If the bolt head is protruding, these Japanese screw pliers work really well (as long as the bolt isn't severely cross-threaded). Around $15. Comes in 3 sizes. But the dinosaur is false advertising (edit: oh it's nickname is M2 NEJI-SAURUS).
ENGINEER INC. - SCREW PLIERS TO REMOVE STUCK, STRIPPED OR DAMAGED SCREW
I've used it for up to M3 bolts and it worked. Although it failed on M3 set screws that were severely cross threaded at a diagonal angle.
