Gog.com (good old games! DRM FREE! lots of games from the 90'sand 2000's for 2.99$ to 9.99$. You can use paypal!)
www.desura.com (moddb and indiedb's own digital distribution platform for buying indie games from indie devs or just to install mods and post on moddb. Ive so far installed alot of free mods and games from it! love it!)
and of course theres Steam, www.steampowered.com. But you already know of that one.