Borderlands 2 STEAM Key $10 from MacGameStore.com
Currently listed as 'Temporarily Sold Out - New orders will be fulfilled soon' but you can back-order.
Ostensibly you're buying for Mac, but of course the Steam key is good for PC too.
Removed non-working coupon
IMPORTANT: If buying for PC, there's some uncertainty about whether a Mac and Mac app is required to claim your Steam key. Before purchasing for PC please read this entire topic on Slickdeals, and if in any doubt wait for another deal to come along: http://slickdeals.net/f/6106672-Borderlands-2-PC-or-Mac-Digi...
Getgamesgo also currently offering Steam keys for $10