The Rokugan d20 books are "produced by AEG under license from Wizards of the Coast, Inc." so I guess they count here.
Campaign Setting:
三y三 of the Sun Eye of the Sun is a first level Shugenja (fire) spell (!) that makes the target save or be blinded and stunned for a round (and they take some light damage if tainted).
B三t三n's Touch Beten's Touch is a second level (Air) spell on the Shugenja and Bard list that gives a +10 (!!!) enhancement bonus to all charisma based skills for 10 mins per level. It can be cast on anyone, though Shugenja are a charisma based class. Wisdom of the Kami is a 4th level Shugenja (Air) spell that gives a +10 bonus to any one skill not barred to Shugenja (though it's still untrained).
Castl三 Castle of Water (OK I'll stop now, but I really don't like the font used in these books using a real and common character, the one for three, in place of E) is a first level Shugenja/Druid (Water) spell that summons a wall of water in a ring around you. Anyone who touches the wall (even if you summon it under them) must save or thrown back, fall prone and take 1d4 damage. It also provides full cover against mundane projectiles (magic attacks, and big projectiles are uneffected while fire damages the wall).
Death of Flame is a 7th level Shugenja (Fire) spell that makes the target save or become feebleminded, confused and slowed for a round per level. I wouldn't note this if the Shugenja's fire list wasn't so crap this looked decent. Similarly Everburning Rage is an 8th level spell that makes the target disabled, explicitly including that they drop to negative HP if they take an action that would do so.
Immortal Steel (Earth Shugenja 4) is PF's Masterwork Transformation, though with an XP cost instead of money.
The Mirror's Smile is a 9th level Air Shugenja spell with "permanent" duration that can't be dispelled (did Permanent duration mean something different in 3E compared to 3.5?) that allows you to change the target into any other humanoid form. They gain the attribute bonuses (not modifiers) and natural abilities of the new form and lose all traits of their original form except bonus feats and skill points (so humans it's a strict upgrade). You can also make them look like the duplicate of an existing person by saying the appropriate full name. No save, but 10 min casting time, so you could use it offensively with a scroll.
Osano-Wo's Embrace is a 5th level Shugenja (Fire) spell that sets the target on fire, making them take 4d6 damage each round. The important part is the target can't take actions that require concentration while on fire from this spell, and it's a fortitude negates so it shuts down casters hard. They also must make a will 20 save or be useless.
The Penetrating Drop is a 7th level Shugenja (Water) spell that makes a "30 foot cube of non-living matter" take 3d10 damage per round that increased by 3d10 every round (3d10, 6d10, 9d10) for a number of rounds per level. Even if hardness is factored in, that will destroy most things (especially if extended). Time's Deadly Hand is an 8th level (Earth) spell that makes objects in a 5 foot radius per level age "hundreds or thousands of years". The intended use is to destroy shit, but I'm sure it would be interesting with wine.
Quiesence of Air is a 1st level Shugenja (Air) spell that creates a sphere around a creature that prevents sound from traveling past it in either direction for a minute per level. Silent casting/movement and immunity to sonic spell in exchange for failing listen checks is a nice trade for a 1st level spell.
Tremor is a Shugenja 3 (Earth) spell that makes everyone in a 100 foot radius area save or be knocked prone. That area is massive and you're explicitly excluded (good thing, since as a medium range spell you'd need to be level 11 to not hit yourself).
Wonderful Origami Furoshiki (Shugenja 3 Water) is yet another temporary bag of holding spell (50 pounds, plus 25 per CL to a max of 45). Worse than most due to ending if open.
Magic of Rokugan:
Unless otherwise noted, all spells are Shugenja only.
Agasha's Kiss ([Earth] 3) lets you temporarily reassign raw pluses of a Samurai's enhanced Daisho to special properties for minutes per level. Bane is the obvious use. Any other ideas?
Alter Mind ([Air] 8) is Mind Rape, Lesser. It's a Dominate Person paired with instant change in alignment for the victim. It's not [evil], but it is considered "dishonorable".
Arrows Flight ([Air] 1) doubles range increment of 50 arrows. Meh on its own, but stacks with a bunch of stuff and thus useful for theoretical optimization.
Awaken the Spirit ([Air] 1, Crane only) gives 6+(2*cl) bonus (max of 20) to any skill (but it's still untrained). I don't think the authors for this stuff knew how useful effortless large bonuses to skills. Flash of Insight ([Air] 1) gives you a temporary rank in any skill for a check. This means you know essentially everything but the most secret (DC32+) stuff with no skill investment.
Balance the Elements ([Void] 6) auto dispels all spells of 4th level or lower on the target and heals it 2d8 damage. It's a touch spell though, and Shugenja are squishy.
Blessed Winds ([Air] 1) gives +4 deflection bonus against ranged attacks and +2 deflection bonus to reflex saves.
Borne by the Wind ([Air] 7) gives Fly to everyone in a 50 foot radius centered on you. Need to remain in the area or stop flying though.
Dispel Slumber (Cleric 2, Shugenja 3 [air]) is an unstopable force that just ends sleep effects. Useless for the low level ones (except the area) but there's some special sleep effects out there that are supposed to need special conditions to end that this just bypasses. Also stabilizes everyone in the area.
Distracting Spirits (Bard 2, Shugenja 2 [Air]) forces (no save) the target to make concentration checks on every spell they cast (10+level of spell they're casting+your CL).
Grandfather's Word (7 [fire]) stuns all enemies in short range if they fail a will save. Again, noting largely because fire list is terrible.
I'll look at H-Z spells and other stuff in book later.
Hurried Steps (Sorc/Wiz 2, Shugenja 2 [Fire]) is haste, but only gives the extra spell. No way this is broken, not in the slightest. Not in the slightest...
The Kami Watch Over Me (Shugenja 6 [Air]) lets you cast a second level spell on another character and have it take effect when they want it to (spending the action needed to cast it). Lasts until discharged, so it can be used to carry spells over between days besides the action economy stuff.
Laughter of the Risen Earth (Shugenja [Earth] 7) costs 5000 XP, but stops you from aging for the duration (5 years, can be extended). Not a lot of ways to do that.
Lessons of the Crab (Cleric and Shugenja [Earth] 4) creates a large pool of temp HP (1d10 per level, max 15d10)
Path Not Taken (Water 5) gives you a quick save once per day.
Scatter the Soul's Flames (Fire 5) forces a target to fort save or be reduced to 1 dex and it needs Heal to remove it.
Spirit of Water (brd, shu [water] 1) grants an extra partial action to a target as an instant effect. Decent if translated to a move action, absolutely insane if it allows casting another spell.
Stand Against the Waves (brd 4, shu [water] 3) grants the target an extra attack at highest BAB for every 5 CL you have for a round, but also gives the target a -5 to hit penalty. Can be extended to last two rounds. Pairs well with Strike of Flowing Water ([Water] 5) which makes the target's touch attacks for a round and can also be extended.
Whispers of the Land (druid range and shugenja 1 [earth]) is actually a good first level divination. It reveals if a named target has been within 60 feet of your location (you can move and concentrate), if they touched anything, and (failing that) when they were last here. Narrows suspect pool and reveals stuff of interest without totally breaking the plot. Problem is you can concentrate for an hour per level on this, which opens up a lot of abuse.
Winds of Agression (Sorc/Wiz, Shugenja 3 [air]) is forces reflex save or be blinded for rounds per level in a large area. Problem is the area is everything between you and the end of medium range, so you're going to need to be clever to avoid hurting allies.
Wisdom of the Dragon ([Air] 6) lets you counterspell for free once a round. Actually pretty nice.
Basically the book has a ton of OP spells. I like how it assumes people capable of casting 9th levels are even remotely common enough to include as many as they did.
Rokuganni Steel weapons and armor cost slightly less than a proper magic item for the same enhancement bonus, though it doesn't bypass dr. Tools are the real prize though: 1500 for a +8 (See the above about undervaluing skill bonuses).
edit: Fortunes and Winds gives alternate, always LA+0, hengeyokai. One serious issue with them is they need to take a bunch of taboos which, unlike the Wu Jen taboos, are actually quite punishing (both the demands and penalties for failure) and many completely screw over an adventurer. They get to pick some special abilities from a point based list. Dragon deserves a note since they need to take fewer taboos and have a good animal as their natural form (medium constrictor snake with fly speed) and can explicitly cast in that form. Aside from dragon, Improved Animal Form (use your human form's ability scores for your animal form and be able to cast in it. With phoenix or crane you're a tiny, flying caster at level 1) and Shapeshift: Imitate (Take the form of a specific human with an alter self effect if you have a cherished possession or part of their body. Only comes online at 12th level, but the ability score shenanigans are worth it)
edit: Dragon 343 has a bunch of feats. Most of them are meh, but it's notable in that their support for Shugenja and Spirit Shaman in there even if it is crap. A few of them caught my eye though.
Spirit Spell lets a Spirit Shaman blow 4 uses of Chastise Spirits as a standard action that automatically maximizes all spells you cast the following round (wording seems intentionally designed to stop quickened spells from being used).
Divine Sorcery is a feat that gives a Sorcerer as domain's power and adds one spell from that domain to their spells known each day. Requires no cleric levels, alignment appropriate for deity and you take the feat the same level you take your first level of sorcerer.
