Super-duper Valve shooter is currently being offered for half-price on Steam.
That means it'll set you back USD 9.99, or just over a fiver in the old money. Well worth it for a game Tom reckons deserves 9/10.
Quick sticks, mind, as the offer ends 1st September. It's still available at the time of typing, probably because 1st September has only just begun in America.