Ok, more PrC run-downs...
Disciple of Baalzebul - 10 levels. You're a liar, cheat, and a thief. You get sneak attack, some bonuses to Bluff, a few spell like abilities (suggestion, etc), some summoning abilities, and some Cha boosting.
Disciple of Dispater - 10 levels. Warlike General of Evil. You have a great affinity for metal (Dispater being in Dis, the iron pit). You get some attack abilities relating to iron or steel weapons, several spell-like abilities related to iron (rusting grasp, a few others), and some summoning abilities.
Disciple of Mammon - 10 levels. You take what you want when you want it, you're quick to betray your friends and allies, and are willing to do embarrassing or awkward things to get what you want. You get several bonuses related to thiefly skills, along with some spell-like abilities to further those aims. They're hard to detect, and can deflect spells. Also summoning abilities.
Disciple of Mephisopheles - Weilds hellfire and all things fire-related. 10 levels. Lots of fire-related abilities - they can throw it, are resistant to it, are better with spells that use it, and can turn himself into it. Also summoning abilities.
Lifedrinker - 10 levels. A vampire's vampire. Vampire PrC. Stores energy (blood) in his Lifewell. He can accumulate points in his Lifewell from using his energy drain attack. He can spend the points to boost spell power or his attacks. Also gets better at draining blood and some more stuff.
Mortal Hunter - 10 levels. PrC for outsiders that hunt mortals, oddly enough. Gets favored enemy, mortals, some shape-shifting, detecting, and smiting abilties, along with several other abilties to make it bad to be a mortal. Also gets a small (paladin-like) spell progression.
Soul Eater - 10 levels. This is basically a soul vampire. After draining a soul (energy drain-like attack) he can power a host of powerful abilities (stat increases, shape-shifting, combat abilities, etc.).
Thrall of Demogorgon - 10 levels. A madman that seeks out the morbid and bizarre. Gets either four bonus feats, or 4 spellcaster levels. Gains some natural armor, several nasty touch attacks (fear, etc), and several abilities related to Demogorgon's dual nature (split personality). Also summoning abilities.
Thrall of Graz'zt - 10 levels. Sinister, conniving, evil master of arcane lore and dark secrets. 6 levels of spellcasting progression. A kind of sneak attack with spells thing. Summoning abilities, and bonuses to Cha-based skills.
Thrall of Juiblex - 10 levels. Oozy, nasty people. You get more ooze-like as you go. You can summon oozy things and also demons. Also get several oozy powers.
Thrall of Orcus - 10 levels. Tool of misery, murder, and revenge. Prefers the company of undead. Like the Thrall of Demogorgon, gets either four bonus feats, or four levels of spellcasting progression. Some summoning abilities. You make yourself more like a demon or undead. Some spell like abilities relating to fear. Bonus vile feats. Some minor shape-shifting abilites.
Ur-Priest - 10 levels. Priests that despise gods. They siphon away divine power and use it for themselves without having to pray to a god. Gain a slow spell progression that goes all the way to 9th level spells. Can rebuke undead, has some SR, and can cause havoc by siphoning away others spells or abilities.
Vermin Lord - 10 levels. People who like creepy crawly things. 6 levels of spellcasting. Some insect-like natural armor, some vermin companions, some spell-like abilities that make him more vermin-like.
Warrior of Darkness - 10 levels. Evil fighter that uses magical rituals to enhance his combat ability. Can make a host of oils and elixers that grant him various feats or combat perks. Also can enhance his weapons, gains some DR and SR.