http://www.gametap.com/home/play/free.html
Gametap has a list of free games that changes every month or so, right now I'm playing Thief, which is pretty fun.
You do have to download a player to be able to download the games, and you do need to register if you want to play mature rated titles, but it's totally free, and I didn't even have to wait for a confirmation email either. Just punched in a name, email and password and I was set.
The downloads can be big, Thief was a little under 900MB, It took me about 2 hours to get the whole thing. It took me a while to get the controls mapped to my PS2 controller, but hey, free is free.