I have a free code for Poker Night at the Inventory (works on PC or Mac) by Telltale Games - it's a pretty fun poker game starring the TF2 Heavy, Strong Bad, Max (from Sam and Max) and Tycho of Penny Arcade. The code must be redeemed at http://telltalegames.com/gifts (I don't know if it works on Steam).
I have one code - first person to PM me will get it. I'll close the thread when it's gone.