This is pretty old news but hey, some of you might not even know about this.

If you have a GPU made by ATI you can get the following games:
Half Life 2: Lost Coast
Half Life 2: Deathmatch
Redeem: http://www.steampowered.com/ati_offer1a/

If you have a GPU made by NVIDIA you can get the following games:
Half Life 2: Lost Coast
Half Life 2: Deathmatch
Portal: First Slice
Peggle Extreme
Redeem: http://www.steampowered.com/nvidia/