RAGE, id Software's post apocalyptic shooter, can be played for free this weekend on Steam, and if you want to own it forever you can grab it for just $4.99. You have until Sunday at 1:00 PM Pacific time to play it for free, and the sale runs until Monday. If you already own RAGE on Steam, you can get The scorchers DLC for $1.24 as part of the sale.
I played a couple of hours of RAGE with a friend on his Xbox 360 a few years ago, and I wasn't that impressed with it. It does look really pretty though, and it's hard to pass on games for five bucks. I'll most likely just check it out over the free weekend to see how it is on the PC.