The NPC class adept is considered a tier 4 class, so it's theoretically more versatile than tier 5 classes like the fighter, monk, paladin, soulknife, and so on, and the class is a reasonable choice for a PC caster in a low-tier campaign. However, I've not seen any material that makes adepts any better except stuff that improves familiars and the prestige class hexer from Masters of the Wild. (Entering the prestige class requires that a creature be able to cast lightning bolt as a divine spell, and the adept is the only SRD class that can do that.) Even though they're tier 6—like the commoner!—, there's material devoted to the aristocrat NPC class like The Quintessential Aristocrat. But, apparently, no material devoted to the poor adept.
Searching the Web for help with adept is tough: results tend to be about the green star adept, dragonfire adept, and other classes with adept in their names instead.
What additional options are available to the adept? I'm interested in answers like those to this question
P.S. Both D&D 3e and D&D 3.5e material are acceptable. Official material preferred but content from licensed material is acceptable as is third-party material.
Best Answer
The Eberron Campaign Setting introduces the religious adept
The adept's spell list is expanded a little by a variant adept presented in the Eberron Campaign Setting on—appropriately enough—Adepts:
This allows some significant variation between otherwise generic adepts.
Other texts add a few spells
The following spells are added to the adept spell list by other sources:
There are probably a few other extra spells added to the adept list floating around in other texts, too, but this is what I had at hand.