It has come to my attention that TONS of steam games are reduced in price by 50% or more. I spend most my time in Quakelive, but am also interested in casual other steam games...

So can anyone recommend some hidden gems / obvious buys to someone who isn't fully aware of what to look for? Arma II looks great and is only 6 or 7 dollars currently. Also dayz has always looked so fun but now that the standalone is out I don't know if I should just wait on that.