In metal, the only modes you pretty much need are Aeolian, Dorian, and Phrygian. But here are all of them.

Ionian - The basic major sound. Think 'happy' or 'triumphant'.
Dorian - The all-in-one blues, rock, and metal scale. This one's great for a jam in minor.
Phrygian - The exotic diatonic mode. Sound a little middle-eastern/Egyptian. Used frequently in riffs.
Lydian - The Vai mode. Creates a dreamy atmospere. Very hard to master.
Mixolydian - The Satch mode. Used in acoustic blues a lot, and good for guitar rock instrumentals.
Aeolian - Straight minor; think 'sad' or 'depressing'. Used in classical.
Locrian - Used a lot in metal riffs for that really EVIL feeling. I don't think I've ever heard it in a solo.