Lil' Loogie
ECL: 15
Race: Murkdweller
Classes: Incarnate 1/Totemist 2/Barbarian 2/Totem Rager 10
Feats: Deadly Spittle (1st), WP Finesse (racial), Spit Venom (3rd), Extra Rage (6th), Expanded Soulmeld Capacity (9th)
Details: A simple little build worth explaining. Starting out at 1st, you have a 30 ft. ranged touch spit for 2d6 acid, a 15 ft. cone of blindness, a 25 ft. touch attack for blindness, and a 15 ft. cone of 2d6 acid at your disposal. By 3rd, you add a 1d3 Con spit. The Barbarian levels grant more DC ups for your acid and poison, in addition to some combat viability. You can invest in nice ranged soulmelds, so inspect the list well.
Death Spit
ECL: 20
Race: Azurin
Classes: Incarnate 20
Feats: Deadly Spittle (1st), Expanded Soulmeld Capacity (human), Point Blank Shot (3rd), Necrocarnum Acolyte (6th), Improved Spit (9th, 12th, 15th, 18th).
Details: Nothing complex, but a pretty solid build for a weak class. At 1st level, you've got a 3d6 cone of acid 15 ft. long. Things get FUN when you get Throat Binding. With an Incarnum Focus and Expanded Soulmeld Capacity (+2 from the class, +1 from the feat), you're dealing 9d6 acid the first round, then 13d6 every round after, in a cone up to 75 ft. long. Basically, it's a cheap way to give the poor Incarnate SOMETHING useful to do in combat, though it's mostly only exciting at low levels (later on, you're better off playing something else, honestly). Feel free to throw in 10 levels of Necrocarnate if you'd like it to be a bit tougher, as they can get you some zombie meat-shields, unlimited essentia, and a whole crapload of temporary hit points if you play your cards right.
The Holy Hawker
ECL: 20
Race: Azurin
Classes: Cloistered Cleric 1/Incarnate 2/Cloistered Cleric +2/Sapphire Hierarch 10
Feats: Spit Venom (1st), Deadly Spittle (human), Animal Devotion (Domain feat), Knowledge Devotion (Domain feat), Extra Turning (3rd), 6th - 18th feats FREE.
Details: This is one elegant little honey of a build, and has absolutely no holes in it. It grants 13th level Clerical casting, 12th level shaping, and you even start tough, with a 1d3 Con damage, 15 ft. cone of poison and a 30 ft. touch attack loogie. You grab an Acid backup by 3rd level, and can start doing whatever you like from there on out. Rock solid, and one of the more reliable and level-appropriate builds to spit with.
Spit-Storm
ECL: 20
Race: Human
Classes: Wizard 1/Druid 1/Incarnate 1/Sorcerer 1/Incarnate +16
Feats: Extra Familiar (1st), Extra Familiar (1st), Share Soulmeld (3rd), Extra Familiar (6th, 9th, 12th, 15th, and 18th).
Details: This is just ridiculous. Get yourself an animal companion and two familiars from the beginning class dips. Until 3rd you'll play like a normal low-level caster; come 3rd level, you'll have 3 familiars and one animal companion who, all of a sudden, can spit just like you. Thus at 3rd level you're talking you + 4 pets, all spitting 2d6 acid globs for a lovely 5x2d6 = 10d6 touch attack. The obvious choice is a batch of tiny vipers. As your levels go up, you score more pets, so that at 6th you're dealing 6x4d6 = 24d6, at 9th 7x4d6 = 28d6, at 12th 8x5d6 = 40d6, at 15th 9x5d6 = 45d6, and at 18th 10x7d6 = 70d6. If you add on an Incarnum Focus Amulet, that'll go up to 80d6; if you drop a tiny viper and grab Expanded Soulmeld Capacity, you'll go from 10x8d6 to 9x9d6 and net ONE lousy d6. Another option, if you want to be REALLY extreme, is to drop one familiar, add Skill Focus - Handle Animal, and drop 10 levels of Incarnate for 10 of Animal Lord. This nets you three more total pets, but you lose one die of damage due to missing out on the 15th level Incarnate boost to soulmeld capacity, so you end up with 13x7d6 = 91d6. Plus, you can afford to have at least one animal companion big enough to ride who probably won't die in every fight. You could also go the Evil route and be a Thrall of Demogorgon or Thrall of Orcus; it costs two feats, but gets four of any kind, netting you two extra pets - the abilities might be more your playstyle than the Beastmaster.
I prefer the Incarnum option just to pursue the class fully; the skill-monkey flavor is fine with a spit-storm as combat contribution. Make sure to buy a bunch of Familiar Pockets for your familiars, since they hurt you when they die (there's another early Dragon feat for gnomes out there that grants animal companions, instead, but you'd lose your human bonus feat, and lose one net pet; on the plus side, you'd always be able to replace any dead pets without losing exp). The idea's simple: wear a big cloak. When you go into combat, a whole crapload of snakes peek out and pelt the enemy with loogies (you do, too). If you REALLY want to get silly, get Great Fortitude and Poison Healer (costing two pets) and, of course, have a ridiculously high Con. Now, if you're hurt in combat, you can have all your tiny little vipers bite you to heal Con Mod x pets in HP. A final fun note about this build: it's great never having to open doors. You'll melt a tunnel through any dungeon you encounter.