Newsarama provides a long list of them

But here are the ones I like the most

Connor Kent aka Superboy

Part Luther, part Superman.  All you need is black shirt, a red superman logo, and some blue jeans.

Death (from Sandman Vertigo Comics)

Just go to Hot Topic and Goth it up.  You have to get the ankh.  The ankh is key.

Scott Pilgrim and Ramona Flowers (Scott Pilgrim Saves The World)

seems like a good couples costume.