For limited time (already lasting for few years), Steam offers ATI and Nvidia users who will register new free account the games "Half-Life 2: Deathmatch" and "Half-Life 2: Lost Coast" for free!
This will also let you download third party mods like "Insurgency", which is even better then HL2:DM in my opinion, and "Dystopia", which require you to have source engine game installed. (haven't checked it)
Even if you already got Steam account with these games, I would recommend registering new account to take the offer. Later you will be able to give this account to your friends who doesn't have these games to play together.
Here are the links to the offers:
Nvidia offer
ATI offer
And here's a video of Half-Life 2: Deathmatch