The spellthief is basically a cross between the rogue and the sorceror. He doesn't have quite as many skill points as a thief, and misses out on a few of the thief abilities, but he gains the ability to cast spells around fourth level.
The spelltheif gets sneak attack (although a little slower than a true rogue) and he can choose to give up part of his sneak attack in order to steal a spell from his target. He can then cast that spell himself, or if he's above fourth level, use that energy to cast one of his own spells. As he goes up in levels, he gets the ability to steal energy resistance, steal spell efects (so he borrow a foe's bull's strength), and steal spell-like abilities. That last one is really neat when there is a warlock in the party, because I can borrow spell-like abilities from willing creatures for free, and since a Warlock has unlimited use of his spell-like abilities...well, I think you can see some of the possibilities.
Daggerspell Mage is a prestige class that give you the ability to cast spells while your hands are still full--providing, of course, that what your hands are full of is daggers. It also grants the ability to deliver touch spells with daggers, and a handful of other neat dagger tricks.
