I just saw this video of a game called "Portal" that will be packaged with Half-Life: Episode 2 for PC. Right now it's strictly a mini-game, but is apparently supposed to give you an idea of things that might be implemented into future Half-Life games. It's pretty crazy.
That might be the most ingenious game-design ever.
I wish it were a full length game in and of itself, but it looks like I might just have to get Half-Life: E2 so I can play it. Maybe it'll get rave reviews and they'll make it into a seperate game someday. Seems like too great of a concept to "waste" on just a mini-game.