Monk: Yes, the lowly Monk. With Hide, Move Silently, Spot, and Listen on the class list, plus all good saves, this guy are capable of doing the job... to a degree. Their lack of trapfinding hurts, and they can't get Mindsight, but their ability to run away quickly can actually be handy at lower levels and they do have evasion to go along with those nice saves. They're not ideal, but they can get the job done in low power games. Consider being Necropolitan to reduce MAD, and dipping something with Trapfinding if possible.
Complete Adventurer Ninja: They've got all the basics of scouting available, including trapfinding and the various skills, plus useful tricks like Invisibility when needed. Lack of UMD means a single Arcane Locked door can foil them, and they lack advanced detection options. Like Monks, they suffer from MAD and thus could really use Necropolitan. Sadly, they can't Ki Dodge in response to a trap being triggered or an unexpected attack (since it's a swift action), and Ghost Step is a Move Action for some reason, but it could get them out of a few nastier death traps. For some reason they don't get evasion until level 12, which really hurts.
Marshal: What's this guy doing in here, you might ask? Simple: a one level dip into this class lets you add your Charisma to all dex based checks for you and your party, which means all Initiative checks, Move Silently checks, and Hide checks are covered. Note the requirement is that the party can hear you and understand you, so you'd have to be close to the party and whispering to keep them stealthed... but that's still likely better than running around with no ranks. Ask your DM if telepathy lets you do this silently... if so this gets much better. But even without that you can use the boost on yourself.
Expert: Yes, an NPC class can do the job, mostly due to getting all the appropriate class skills. They lack trapfinding and evasion, but what were you expecting from an NPC class? Usually games where you're playing NPC classes also have low wealth which hurts too, meaning their UMD may go to waste. Best of luck.
Spellthief: MAD hurts these guys bad, and their skill list isn't as impressive as some of the other dedicated scout classes. But they do qualify for Mindbender and Earth Dreamer, they're amazing with Shadowcraft Mage, and they've got some spells to work with. The Master Spellthief feat is critical if you do anything other than pure Spellthief. Though low power in general, these guys can be part of some serious nasty builds... consider Spellthief 9/Mindbender 1/Shadowcraft Mage 5/Earth Dreamer 5 with Mindsight and Master Spellthief.
Warlock: Lack of Hide and Move Silently really hurts these guys, but they do qualify for Mindbender and they can have all day invisibility/flight available. Detect Magic at will is handy too, as it can find magical traps, and their UMD and item creation abilities are nothing to scoff at. See Invisibility is handy too (though Detect Magic usually handles this), and so is their teleportation. You're probably going to want to Multiclass to really make this work.
Scout. Like Rogues, Scouts have lots of skill points to work with, but they lack UMD which means they're stuck when dealing with many magical things. They do have more Knowledge skills to work with, and a few solid minor abilities that help (such as boosted movement speed and initiative). Still, despite the name, they're by no means the best in this category. Hide in Plain Sight comes rather late for them and while they do get Blindsense/Sight eventually, the limited range means its of little use.
Rogue. The classic scouting class, these guys have a solid number of skill points to get the job done, and have all the basic scout skills covered except for Knowledge skills. Evasion is solid, and so is their eventual ability to take 10 on all skills. UMD is their only way of getting around Arcane Lock, but at least they have it. The class even has Defensive Roll, which can function as an ablative defense (though it comes rather late in the class to be of serious use). Unfortunately, they have a tough time standing up to a single attack that doesn't target reflex saves due to weak HD and generally poor AC, but dips into other classes can help with that. Consider also the Wilderness Rogue variant, which adds the Ranger's Hide in Plain Sight to their special abilities.
Ranger: They've got some useful skills for the job and generally more hitpoints and armor to work with. Plus, they naturally combine well with the Scout class, and they do get Hide in Plain Sight eventually. They're one of very few scouting classes with full BAB, and while it's late they do get Evasion eventually. If you want to be a scout, this is a class worth playing with a bit. Scout 4/Ranger 16 is a very natural scouting build. Also consider the Wild Shape variant, which adds much needed mobility and versatility. Note that they have ACFs to gain Trapfinding if necessary.
Bard: Though they lack trapfinding and a bunch of other important skills, the Bardic Knack ACF can really make up for this and their social abilities are impressive indeed. They can also easily get into Mindbender, so it's possible to get the job done with these guys.
Beguiler: A true Int based class, Beguilers will regularly have more skills to work with than Rogues, and bring magical power into the mix. Their ability to dip Mindbender easily for Mindsight is a huge boon, and there's a few other really solid arcane PrCs they can use to dramatically increase their abilities (Earth Dreamer and Shadowcraft Mage come to mind). Low hitpoints and defenses are a danger to worry about, but their spells usually make up for this... Mirror Image is great, and Silent Image can trick enemies into attacking the wrong you entirely. Solid Knowledge abilities really help here.
Binder. Binders can absolutely rock the scouting role, even at level 1 if they took Improved Binding. Malphas can give them unlimited birds that scout for them, triggering traps safely. Note that these birds can still trigger alarms and alert enemies to your presence so it's not entirely without risk, but they're still really handy. You also get the ability to go invisible for short periods starting at level 5 (before Invisibility starts becoming useless due to Blindsense and the like). Note that while you do get some nice knowledge skills, you lack Hide, Move Silently, Spot, and Listen, so you'll need to dip other classes if you plan to scout with anything other than birds. Other useful vestiges to consider include Acererak (Detect Undead and Hide From Undead at will), Balam (reroll one save or skill check every 5 rounds, insight boost to AC, initiative, and reflex saves), Chupoclops (Ethereal at will though you can't move while doing so, short range Blindsense/Deathwatch ability), Dantalion (+8 Knowledge Checks, thought reading at decent range, short range teleportation, a no save "don't harm me" effect), Haures (Immunity to all mental detections including Mindsight and free Mind Blank, Incorporeality at will, Major Image at will), Karsus (Autodetect all magical auras within 30 feet, Dispell Magic at a touch, use Wizard magic items), Leraje (+4 Hide, Superior or regular low light vision), Marchosias (+16 Hide and Move Silently, Gaseous Form at will, Death Attack), Naberius (Disguise Self at will, Diplomacy and bluff as standard actions while taking 10 even when threatened or distracted, command enemies at will, heal con damage which works amazingly with the Ritual of Shadow Walking), Orthos (Blindsense 30, 50% miss chance, messages via Whispering Wind), Otiax (Concealment at will, Open objects at range, unlock any door), Shax (Freedom of Movement at will, swim speed), and Tenebrous (Deeper Darkness at will, See in Darkness, Flicker a few times per day). Yeah, there's lots here. Plus the Anima Mage PrC can be quite good.
Swordsage: Lack of trapsense hurts, and you're missing some useful skills. But this class multiclasses extremely well... a two level dip in Swordsage (preferably the Unarmed variety) is recommended for almost any scout build. Shadow Hand provides amazing ablative defenses and maneuverability tricks like Shadow Jaunt, while Diamond Mind can guarantee you always pass a save when you need to, and even Stone Dragon lets you punch through walls without trouble (great for escaping certain death traps). Nothing surprises a DM the first time like his nifty monster charging the scout... only for it to be thwarted by Counter Charge and then chucked right back into the party with Mighty Throw. The initiative bonus this class gives can't hurt either. They even get Evasion, though it's a bit late in the class to be of serious use. If you're going to dip Swordsage, wait until level 5 to do it so you can get second level maneuvers to start. The most obviously useful scouting maneuvers include Moment of Perfect Mind (succeed at a will save), Counter Charge (one charge against you fails utterly), Action Before Thought (succeed at a reflex save), Cloak of Deception (turn Invisible for a round, combos great with Shadow Jaunt), Shadow Jaunt (teleport 50 feet as a standard action, though note they can see you do it if you didn't go invisible already), Mountain Hammer (ignore DR and Hardness and do an extra 2d6 damage, great for smashing through objects), and Mind over Body (succeed at one fort save). There's more at higher levels if you want of course, but that's the basics. If you're taking two levels, make sure you have one combat stance and one non combat stance... I like Assassin's Stance for combat (+2d6 Sneak Attack) and Hunter's Stance out of combat (Scent). Dance of the Spider gives Spider Climb for free and is worth considering, but you could likely get climbing (or flight) via items instead, and Child of Shadows is also worth considering as it gives a miss chance when you move.
Factotum. An Int and Dex based class that adds Int to all Dex and Str based skills, this class is the master of all skills and the quintessential scout. Brains over Brawn also gives Int to initiative, making this one of the highest initiative classes on this list. You have enough magic to get through basic situations, and a variety of very useful abilities when you need them. You also have all knowledge skills to work with, which is a handy ability. This class synergizes VERY well with Swordsage. Note that you do qualify for Mindbender so you can take that dip for Mindsight. You'll probably want to either go for an Iajuitsu Focus/Quickrazor based character or a Knowledge Devotion/Archery based character when doing the scout thing, depending on how you spend your skill points. Note that you lack evasion and thus might want a Ring of Evasion. Alter Self is a great spell to have and you've got it by level 5... the ability to turn into a Skulk, Air Mephling, Earth Mephling, Aquatic Elf, or Crucian when you need is really useful. Also consider Craft Wonderous Item or Craft Wand, as you've got everything else you'd need to get the crafting done.
Psion: In addition to their usual powers, a Psion has a psicrystal that can take Lifesense (due to being a construct) and Mindsight (but only 30ft range). Since it can telepathically talk to its owner, it's quite capable of moving ahead a bit and scouting... but note that it only has 40 foot standard vision. Their ability to see in any form of darkness is quite handy as well. Note that this article
grants Telepaths telepathy... allowing them to take Mindsight.
Artificer: You have few skill points to work with but are Int based and have trapfinding, and with items can easily be an amazing scout. With that said, Artificers are so powerful that scouting is just one of things they can do when they feel like it, so it's hard to make an Artificer that actually feels like a scout (it's hard to make an Artificer that feels like he's any specific role... it just feels like he's rocking that particular role right now, but if he made different items he'd be something else). Still, this class can definitely rock the ninja concept if he wants to.
Cloistered Cleric: You'll need Kobold domain to detect traps and Trickery Domain for hide. But you've got the skills and skillpoints, and anything you don't have naturally can be gotten via spells. Like the Artificer, it's hard to really feel like a scout class... you're just a demigod who feels like scouting right now. But if you ever wanted to play a crazed sneaky Kobold who sneaks up on people and then lets down the full fury the gods while yelling "DRAGON SNEAK ATTACK!" then this class will work... technically you can't sneak attack with Miracle but don't tell the Cleric that or he'll smite you.
Druid: The last of the T1 scouts, these guys are hard to make feel like scouts... they're gods who can scout when they want. Still, there's no arguing with the fact that they're extremely good at it, with their high Wisdom scores, useful Wild Shape abilities, and host of handy spells.