http://source.bungie.org/get/
I hope you like being lost.
And motion sick, to a lesser extent, since the "3D" environments bend when you look up or down.
The story is awesome, though, and aside from having the standard "what the fuck was I doing, again?" syndrome common to games with huge levels it's also pretty fun.
I also like the rocket launcher effects in the video. Half the games I play these days don't look that cool, for whatever reason.