I have had the Orange Box for a little while now (of course) but somehow hadnt got around to Portal. Well, no longer - Portal was a real blast. The story, voicework, music and styling made for an amusing and engrossing feel to the puzzles and mind-bending joy of the portal logic.
There way the portals work is really neat, and pretty much every time it works feels cool :-)
[a few screenshots from the Portal Site]
And just for fun, a nice trip back to this promo trailer with the full feel going and some awesome usage of the portals themselves.