Torchlight - Free for 48 hours (until June 20, 20:00 GMT)
They also have a bunch of other old games on sale and some daily deals. The current daily deal (for the next 9 hours from this post) are a 10 game Dungeons and Dragons pack for $21.10, Alan Wake bundle for $4.48.
For people who don't know about GOG, it stands for Good Old Games (or something along those lines) and is known for selling old games (like DOS games and stuff) through PC DDL. The games are all DRM-free and have been modified to work with more current systems.