The rollout of Valve classics to Mac OS continues this week with oldie-but-goodie Counter Strike: Source, which in addition to getting a "major update" is on sale the next two days for $6.80.
That's two-thirds off the normal price. The update delivers new stats and game summary screens, plus more than 140 achievements. The game adds an end-round MVP and a death cam, in addition to the standard batch of game and Steam features.
The game offers PC vs Mac cross-platform play, per the norm. Same ownership rules, too - if you own the PC version on Steam, you own it on Mac.
Counter Strike: Source [Steam]