One more classic Steam title has been added to the Mac lineup -- Day of Defeat is now available on the OS X version of Steam. It's a Source-engine WWII multiplayer shooter that's not quite as popular as Counter-Strike or Team Fortress 2 (both of which are also available on the Mac at this point), but still provides some Axis and Ally-style fun. Personally, I think I prefer Enemy Territory, but Day of Defeat makes use of the Steam Play feature, so if you already own the game on Windows, you can download and play it on your Mac without paying any extra. If you've never played it before, you can pick it up for just $10 or (even better) buy it along with a few other classic Source titles for $30.
That just leaves the Left 4 Dead line as the last big Valve title yet to be released on the Mac. They've been pretty faithful with their weekly releases, so by this time next week, you could finally be killing zombies on the Apple OS. Stay tuned!