Steam Offers 

Hardware Promos (Only work if you have a Graphics Card from a specific Manufacturer)

ATi Promo: Half-Life 2: Deathmatch and Half-Life 2: Lost Coast for free! | Click to redeem | ATi Graphics Card owners ONLY.
nVIDIA Promo: Portal: First Slice (Portal 11-level Demo), Half-Life 2: Deathmatch, Half-Life 2: Lost Coast and Peggle Extreme for free! | Click to redeem | nVIDIA Graphics Card owners ONLY.

Sam and Max: Episode 4 (Abe Lincoln must die)

Free for everyone | Click to install

TrackMania Nations Forever

Free for everyone | Click to install

Half-Life HD

Half-Life High Definition pack | Click to install
Half-Life High Definition pack | Click to uninstall