On Portals and cake..er..coffee
I have been playing Steam's Portal and can only second all that has been said before. Great game, too short. The evil mastercomputer is a great character, the "test chambers" are lovely, the whole portal technology is just fun and the end of the game simply has style.
Beaming was yesterday, now you shoot portals. There are youtube videos showing off the action, e.g. here, an early show-off which conveys the spirit pretty well.
Placing the portals conveniently, you put yourself into an endless fall. That way you get the "falling 30'000 feet" achievement.
Also, gaming would be almost unthinkable in certain circumstances if not accompanied by a steaming cup of coffee.