My gaming group is currently playing a game in 3.5e, but I have done some of the DnDnext playtests and find myself very intrigued by the wizard class' at-will cantrips, and I was wondering if there was a way to build a 3.5 character with at-will spells. I've toyed around with some Sorcerer powers that increase spell slots, but I haven't come up with anything that truly allows for at-will spells.
Being able to actually cast an unlimited number of spells is--barring excessive shenanigans via things like using plane shift to go to planes wherein time passes at a far faster rate so you can rest in the middle of an encounter--impossible. Having a character with a host of at-will spell-like abilities is possible with some effort.
If the character starts as a humanoid, he can take the templates half-fey (Fiend Folio 96) with at-will charm person for LA +2, half-rakshasa (Dragon #313 96) with at-will empathy (Dragon #313 94) for LA +3, and katane (Dragon #313 64) with at-will spider climb for LA +4, then top off with either half-celestial (MM 144) with at-will daylight for LA +4 or saint (BE 184) with at will bless, guidance, resistance, and virtue for LA +2. The character'll have 1 HD when he takes his 1st class level and count as a level 12 or level 14 character, but his ability scores will be amazing.
Also, this assumes reflavoring the names so the character isn't 3/2+ something.
Play a monster. All of the following monsters get at least a few spell-like abilities at will: the ECL 20 astra deva (MM 11) and trumpet archon (MM 17); the ECL 19 hezrou (MM 44); the ECL 18 vrock (MM 48); the ECL 16 couatl (MM 37) and erinyes (MM 54); the ECL 11 hound archon (MM 16); the ECL 12 succubus (MM 47); the ECL 10 drider (MM 89). You can probably tell where I stopped going through the Monster Manual, but picking a monster is almost always suboptimal to picking an actual non-LA, non-ECL character.
The warlock from Complete Arcane's been suggested, but I also recommend Tome of Magic's binder, whose vestiges can grant spell-like abilities at will, like the Acererak vestige which grants at-will use of detect undead and speak with dead. The bound vestige persists for 24 hours after which it must be bound again, so it's not technically inherent-forever spell-like abilities. (Thus foiling your scheme to create a perpetual motion machine via an enslaved creature with an at-will spell-like ability.)
A 5th-level spellthief can steal another's at-will spell-like abilities all day long. The spellthief only gets one use of it before he must steal it again, but if the source is limitless so is the spellthief's ability to steal. Improved familiars make good targets.
Other than Complete Arcane's feat Innate Spell--which lets you trade mind blank or polymorph any object once per day for mage hand at will--getting at-will spell-like abilities via feats looks impossible.
However, like everything in D&D 3.X, being a caster and ruthlessly pushing the limits of the feat Craft Wondrous Item (PH 92-3) pretty much lets you make anything your DM allows. With the core rules setting a precedent with hand of the mage (DMG 258), convincing your DM to allow crafting other comparable at-will items should be trivial.
There's one feat, though, that I mention here because it's the Feats section: Fiendish Summoning Specialist (PlH 39). You needn't take it; just know it exists.
Depending on the errata the campaign uses the spell polymorph any object might be used to just become a creature with spell-like abilities, but neither magic jar nor shapechange grants access to a creature's spell-like abilities.
What does explicitly grant access to a creature's spell-like abilities is the 5th-level Sor/Wiz spell fiendform (SpC 90). It reads, "[Y]ou can take the form of any fiendish creature, demon, or devil that can be summoned by a summon monster I, II, III, IV spell..." (emphasis mine). So if the Fiendish Summoning Specialist feat exists in the campaign, by a strict reading, other folks have added to the summon monster lists, and those can be accessed by anyone who casts fiendform. The fiendform spell only lasts 1 minute per caster level and has as an unpriced but weird material component, and you've to convince your DM that other people totally put the demon, devil, or fiendish creature you want to change into on their summon monster list via the Fiendish Summoning Specialist feat, but if those obstacles are overcome--and you don't mind that sick feeling you get after having so much cheese--, have fun with your demonic and devilish at-will spell-like abilities. If being such a creature all day is required the fiendform spell is a valid target for the Persistent Spell feat (CAr 81) and multiple ways exist to reduce what would otherwise be an 11th-level spell to something that can be cast pre-epic.
There is not, to my knowledge, an identical opposite celestialform spell.
Complete Mage has a set of feats that provide, as long as you have certain spells memorized, you can do at will magical tricks based on the spell memorized. These feats relate to various keywords, such as fire, electricity, or teleportation. For instance, if you take the fire related feat, you get the at will ability to conjure a burst of flame that does 1d6 damage per level of the highest level fire spell memorized. For the electricity feat, it does 1d6 damage per level of the highest level electricity spell memorized. May want to look into that.
Complete Arcane has the Warlock
The Warlock from Complete Arcane is basically the at-will version of the Sorcerer. You get a ray that deals damage sort of comparable to Sneak Attack (i.e. 1d6 every other level, except at high levels it slows down for no apparent reason), as well as a dozen “invocations,” which are at-will arcane spell-like abilities.
Ultimately, the Warlock was badly over-charged for the privilege of having things at will. Most of the invocations aren’t especially awesome to be able to do constantly (though the flight, invisibility, and teleportation ones are pretty sweet), and in combat actions are usually a much tighter limit than spells per day. And invocations are much weaker than spells, at the same class level. But you can still do some pretty neat things and it does make you pretty independent and reliable.
As a side-note, the Dragonfire Adept from Dragon Magic uses similar mechanics, though he’s less “magely,” and some other alternative magic systems (meldshaping, pact magic, truenaming) are at least sort-of at-will, though again they differ quite a bit from your typical mage.
Lastly, there is a grossly-overpriced feat that can allow high-level regular spellcasters to use a select, low-level spell at will. I cannot recommend it.
If your character has written scrolls for all known spells, you achieve the same effect: being able to cast any spell you know at will, without having to bend the rules and without having to be a special class.
But it costs XP and GP to do so; plus you need the Scribe Scroll feat.