Portal was definitely my favorite part of the Orange Box and it ended much to soon. The game continues however in 2D. A couple of guys from We Create Stuff decided they would make a flash version of the excellent Valve game back when it was first announced. Obviously the puzzles aren't as intense as the 3D version, but a lot of the same designs from the original version stay intact. Great way to waste the day.
Check it out, here: http://games.ign.com/index/freetoplay.html/