HD Pack - HL
steam://installaddon/halflifehd
steam://removeaddon/halflifehd

Codename: Gordon
steam://install/92

HL2: DM and Lost Coast (ATI)
steam://hardwarepromo/305

Portal, HL2: DM, Lost Coast, and Peggle Extreme. (Nvidia)
steam://hardwarepromo/609
http://www.steampowered.com/nvidia/#

Sam and Max: Episode 4: Abe Lincon must Die!
steam://install/8230