Adventure

GamesRadar.com has a list of great commercial games that have been released for free.  Included are some of my favorites like Archlord, Battle Chess, Fallout, and many more. 

From the site:

We like free stuff. That’s why we posted links to download and play 365 free games so you could have a decent game to play for every day of the year, free of charge. Unfortunately, GamesRadar has yet to wrestle the most formidable type of free games into submission so that they can be preserved in their proper list form for eternity. We’re talking about the free games that were good enough to sell, important enough to review, and memorable enough to warrant a second play through years later.

http://www.gamesradar.com/f/30-commercial-games-released-for-free/a-20100202122153157099

Deus Ex is now free over at Gametap

Enter a world where the future is always uncertain...a world where the key to survival is held by a chosen few, and where terrorism is fueled by intense desperation. As operative JC Denton, you have a difficult decision to make: do you complete your mission and ensure the safety of the rich and powerful, or do you rebel for the sake of the millions who would be doomed without you?

Link Here

Some other great first-person shooter games that are free are:

• Hitman: Codename 47
• Hitman 2: Silent Assassin
• Hitman: Contracts
• Thief: Deadly Shadows