Valve have released their very first game on Linux and OSX for the first time. The two new versions are listed as being betas though and are not mentioned on the game's store page yet. Regardless, you should still be able to install and play the game on your Macs and Linux boxes now if you already own it on Steam. If you don't already own this classic shame on you, but it's only $9.99. Windows version of the game was also updated so that Gearbox's high definition model pack is now the default set of models used by the game.
Update: Counter-Strike 1.6 has quickly followed Half-Life and launched a beta on OSX and Linux as well. This means two of the most-played games on Steam, Team Fortress 2 and Counter-Strike, are now available on Linux and hopefully Dota 2, Counter-Strike: Source, Counter-Strike: Global Offensive and the rest of the Source and GoldSrc games will follow suit soon.