The Humble Store
Most of us know and love The Humble Indie Bundle...

The place for conversation that doesn't make sense to talk about in any specific game in our list. Think of General Discussion as a "catch all" for conversations about gaming, game culture or anything else worth sharing.
Green Man Gaming (http://www.greenmangaming.com/) is a pretty decent site as well. They almost always have a 20% off coupon code available, and they sometimes throw out a 25% code (and/or sales on select games) that stacks with it. I've been able to purchase XCOM: Enemy Unknown, Dishonored, and the new Tomb Raider from the site for less than I could find anywhere else (including Steam sales) at the times I bought them.
Awesome, thanks for the tip. Checking them out now.
Also http://www.indiegamebundles.com/ will show you all the active bundles across a bunch of sites. I've bought several off http://www.indieroyale.com/ and http://groupees.com/
Deals aren't strictly limited to PC though... I've picked up great console titles at Target on clearance for silly pricing (not quite bundle by definitely < $10). You just have to check the clearance each time you drop by. Also, the selection is weird, so are just overstock so it's surprisingly new stuff, and some is much older... But it's fun to find a gem :)