It's the holiday season, which means practically every retailer's going nuts with sales, and downloadable games are all the rage these days.
Right now, Steam's doing a "Holiday Wishlist Sale" wherein a new game goes on sale every day at 10 AM PST. If you visit the deal page on a given day, you're entered to win the top 10 games on your Steam wishlist. The deals have all been pretty solid, with a good 50-75% off popular titles every day. No doubt as Christmas approaches, and even past Christmas into New Year's, another massive sale will go on at Steam. It's worth it during late November and through the first of the new year to refresh the Steam store to see what's on sale today.
GoodOldGames is running a massive sale as well, on a whole bunch of fun titles. They focus more on DRM-free classic games, from the Windows 95 and up era. If you missed out on a PC game that's been out of print for years, it's a good place to check. Most of their games are 50% or more off through January 2nd, so get on that if that's your thing.
Humble Indie Bundle # 4 is in full swing as well, and this bundle's offerings could be considered the best yet. They're throwing headlining act Super Meat Boy in with Bit.Trip Runner, Gratuitous Space Battles, Shank (a co-op side scrolling beat-em-up) and Cave Story+ (a Metroid-style retro shooter), and then mixing it up with some lesser known titles like Jamestown and NightSky. It's pay what you want pricing, so you can get the whole group for around $5 if you so choose. You can also opt to donate part or all of your payment to charity. They'll even give you codes you can use to add them to your Steam library, although you're welcome to grab DRM-free copies of every game for Windows, Mac, and Linux. It's a total win-win for everyone involved, so there's no reason to not pick this up.
These days there's a million competitors for both Steam and the Humble Indie Bundle, but I really haven't found anything that compares with those two in terms of selection, price, and in the case of Steam, ease of managing your library.