While Linux gamers already have Counter-Strike (sans CS:GO but that's coming soon), Half-Life, and many of the mods for these popular Valve titles, only released on Thursday was Half-Life: Source and Half-Life Deathmatch: Source. With the Linux port comes the first OS X ports of these older Valve titles too.
Right now these games are considered beta ports for Linux and Mac OS X and require the interested gamers to opt into the Beta version of Half-Life 2 to test them. Half-Life: Source is a digitally remastered version of the original Half-Life game and switched to run on the Source Engine. The cost of the game is $9.99 USD via the Steam Store. Half-Life Deathmatch: Source is a similar incarnation of the original multi-player aspect of Half-Life.
More details on the Linux availability of Half-Life: Source can be found at SteamCommunity.com.