Portal is, simply put: the best game of 2007. Possibly even 2008.
What was originally intended as a minigame for the Orange Box, has completely shocked everyone who tried it out.
You wake up in Aperture Laboratories, being informed by a computer voice that you are now an unwilling particpant in the new "portal" experiment. You are given a gun that makes said portals. With the right button you can make an entrance portal, the left creates an exit. You are then herded thoughout the lab's obstacle course, taking your puzzle solving skills to the limit as you try to escape the various deadly traps laid for you using your gun.
Much of the game's winning personality can also be attributed to the voice of GlaDoS - the computer who accompanies your various stunts with her deadpan observations. Trust me when I say that GlaDoS's song will be stuck in your head for days after you hear it.
The game itself takes only a few hours to beat. Thi may seem short, but Portal makes perfect use of the time, and you are not left disappointed with the fact it was so short. To be honest, the only thing you're left wanting is a sequel that can live up to this wonderful game. (Interestingly enough, rumor has it that Valve is already working on a sequel!)