Richard Jones' Log: The cake is a lie
Play Portal. I've previously been excited about games before their release, but never have I been so pleasantly surprised by an unknown game. Game of the year for me (narrowly beating out Team Fortress 2). You don't necessarily have to take my word for it - a lot has already been written, drawn and spoken (fast & loose language warning on that link) by others about the game.
It's certainly one of the funniest games I've ever played with superb writing and pacing.
I just can't seem to get it out of my head :)