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Acolyte of the Skin - Replacing one's own skin with that of a living demon is a goal that most couldn't even concieve of, let alone consider.  This, however, is exactly what some spellcasters seek in their desperate quest for greater power.  From Tome and Blood Requirements:
Alignment: Any non-good
Knowledge (the planes|Outer Planes):  8 ranks
Spellcasting: Ability to cast 3rd level arcane spells
Special: Must have made a peaceful contract with a summoned, evil, outsider.

Acrobat - The acrobat, like the bard, is an entertainer.  His focus is on the athletic aspects of entertainment.

Requirements:
Balance: 8 ranks
Jump: 8 ranks
Tumble: 8 ranks
Feats: Dodge, Mobility
Alienest - Alienists deal with powers and entitities from the remotest and most terrifying reaches of space and time.  For them, magical power is the triumph of the mind over the boundaries of dimension, distance, and, quite often, even sanity. From Tome and Blood Requirements:
Knowledge (Arcana): 8 ranks
Knowledge (The Planes): 8 ranks
Feat: Alertness
Spellcasting: Ability to cast at least one divination spell and at least one summoning spell of level 3 or higher.
Special: Prior contact with an alienest or pseudonatural creature.
Ancestral Avenger - Elves who are fired in the oven of hatred, and cooled in the breeze of eons of experience fighting their enemies, the Ancestral Avenger's sole purpose in life is the extermination of the drow. From Dragon #279 Requirements:
BAB: +5
Wilderness Lore: 3 ranks
Feats: Alertness, Iron Will, Tracking
Arcane Archer - Master of the elven warbands, an arcane archer is a warrior skilled in using magic to supplement her combat prowess. Requirements:
Race: Elf or Half-Elf
Base Attack Bonus: +6
Feats: Weapon focus (any bow other than crossbow), Point Blank Shot, Precise Shot
Spellcasting: Ability to cast at least level 1 arcane spells.
Arcane Devotee - Some arcane spellcasters are also very devout in their faith.  They willingly subordinate their own causes to those of their patron deity, becoming an arcane devotee.  As their magic compliments the divine magic of the faith's clerical leaders, arcane devotees are among the most respected members of a deity's following. (from FR) Requirements:
Knowledge (religion): 8 ranks
Spellcraft: 8 ranks
Feats: Enlarge spell
Alignment: Same as deity's alignment.
Patron: An arcane devotee must have a patron deity and it must be the deity of whom she is a devotee.
Spellcasting: Ability to cast 4-th level arcane spells.
Arcane Trickster - Arcane tricksters combine their knowledge of magic with a taste for intrigue, larceny, or just plain mischief.  From Tome and Blood Requirements:
Alignment: Any non-lawful
Decipher Script: 7 ranks
Disable Device: 7 ranks
Escape Artist: 7 ranks
Knowledge (Arcana): 4 ranks
Spellcasting: Ability to cast Mage Hand and at least one arcane spell of 3rd level.
Special: Must have the sneak attack ability for at least +2d6 damage
Arcanopath - The arcanopath trains in methods to fight and slay spellcasters,  utilizing the Rule of Arcalos.
(from Dragon #281, pg. 36)		 Requirements:
BAB: +4
Knowledge (arcana): 8 ranks
Feats: Improved Unarmed strike, Deflect Arrows, Dodge, Mobility
Alignment: Any lawful
Special: Most find the Monastery of Finithamon, successfully petition the sensei for membership, and have slain an arcane spellcaster.

(Monks who join the order of Finithamon may freely multiclass as an Arcanopath without restriction)
Archmage - The most advanced practitioners of magick, archmages bend spells in ways unavailable to other spellcasters. (from FR) Requirements:
Spellcasting: Ability to cast 7th level arcane spells, knowledge of at least 5th-level spells from five different schools of magic.
Spellcraft: 15 ranks
Knowledge (arcana): 15 ranks
Feats: Spell Focus in two schools of magic, Skill Focus (spellcraft)
Assassin - Master of dealing quick lethal blows, the assassin also excels at disguise and infiltration. Requirements:
Alignment: Any Evil
Move Silently: 8 ranks
Hide: 8 ranks
Disguise: 4 ranks
Special: consult your DMf