For two days the price of the game gets dropped to help win over the masses. CS: Source released back in '04 and has been super updated.
Valve have chopped the original price of £13.99 to just £4.76 but this sale lasts only two days. They confirm that both PC and Mac games can enjoy "cross-platform play" as well as all the other latest improvements to the engine.
Counter-Strike: Source received a mighty update not too long ago which added in player stat tracking, achievements with Steam, an overhauled scoreboard and more. Enjoy!