I'm sure we have all bought a Humble Bundle or two, Maybe a BundleStars, or even an IndieGala that contains a game that you already own.
So I propose that we have a thread (I looked and I couldn't find one) where you can post what spare Steam/Desura/Origin/Uplay keys you have and see what people are willing to trade for your unwanted keys.
I have
Arma II (Steam)
Dead Space (Origin)
Burnout Paradise: The Ultimate Box
Dead Space 3 (Origin)
Medal of Honor (Origin)
Mirror's Edge (Origin)
Command & Conquer: Red Alert 3 - Uprising (Origin)
Populous (Origin)
Crayon Physics Deluxe (Steam)
Dungeon Defenders + All DLC (Steam)
Nightsky (Steam)
And more to come when I finish going though my lists
So… anybody want something I got? Come make an offer
EDIT (added 2 new games)