Portal is a first person platformer with a fun addition. Like other platformers, you get to run around trying to get from here to there. But it's the future now, and you've got a hand-held wormhole maker!
Can't jump up to the platform? Pull the trigger to open a wormhole next to you, then shoot to place its other end on the wall next to the platform. Step through and poof, you're there! Of course, the levels aren't suddenly a piece of cake, they get very sneaky.
It's a brainy sort of fun. You have to solve path-finding puzzles using more than just what's visible on the screen and more than just three dimensions. Oddly enough, it's pleasantly twisted when the mental map in your head is a klein bottle.