I don't think I've gotten around to ranting about this on here yet (feel free to correct if this is wrong), so a while back I got myself a copy of Portal. For those of you who have been dead for the last 6 months (or plausibly, some non-gamers) Portal is a very simple game at its core. You have the Aperture Science Handheld Portal Device (the "portal gun") which can be fired at most flat surfaces, to create a "portal" on that surface. It's got two firing modes, which create blue and orange portals, and once you've created both anything going in one portal comes out the other and vice versa. You can then use this to do things like fly across rooms because your momentum is conserved between entering and exiting (or, as part of the game dialog puts it "speedy thing goes in, speedy thing comes out").
This single mechanic then used to create a fairly small game - 19 puzzles, plus about an equal amount of time running around the base doing more puzzle stuff, and finally a good ending scene. In all of this, you never encounter anyone else, and it's all narrated by GLaDOS, a snarky crazy bugger of an AI (see Wikiquotes for a decent list). The whole experience was played out by me over the course of about 3 evenings, and according to various reviews they claim its only about 6 hours long, which is pretty short for a game.
Now, despite the limits, Portal is amazing. I can't state this clearly enough. It's a brilliant experience, and this appears to have infected a number of others. Elements like the "Still Alive" song from the ending sequence; the "Weighted Companion Cube" (a big block with hearts on the side you move around in one puzzle); the infamous cake; and "other things you could do with portals" appear to have gripped chunks of the community. Samples of this include:
If you haven't played it already, you really should.