For years now, I’ve struggled with the existence of prestige classes in the d20 RPG system. I understand the desire to have them; they provide an underlying rule set to accompany a significant variation on the core character archetypes that are normal classes. They are “prestigious” in that there are necessary prerequisites to gaining entry into them. They are “classes” because progression in a PrC takes the place of the normal progression through a normal class.
Since the 3rd Edition Dungeons & Dragons Players Handbook was released in 2000, Wizards of the Coast has been on a fairly-steady schedule of monthly releases, putting out additional skills, feats, spells, classes, and prestige classes that can help mold their rule set to whatever setting you may wish to run it in. They’ve re-released the Forgotten Realms and Greyhawk, as well as Eberron and a number of environment-type-based settings (Frostburn etc.).
All told, there are currently 652 prestige classes published by Wizards of the Coast for the d20 system. This doesn’t count the plethora of prestige classes present in OGL products like the Iron Kingdoms, Midnight, or Warcraft. Granted, many of these 652 PrCs are repeats (Archmage, Incantrix, and Wayfarer Guide are some that show up in two books), but this is a truly-obscene preponderance of customized character rules, and likely indicative of either a fundamental flaw in the system’s underlying mechanics or something far more insidious in their marketing department.