Portal
I've been looking forward to Portal for awhile, and was one of the reasons I bought the Orange Box thing. Anyway, I started playing it when I got home, and finished it a little while ago. Fun stuff. When the first portal trailer came out, I found the team's first game that got them hired, Narbacular Drop. Narbacular drop was about 15 minutes long and INCREDIBLY glitchy, but it was still pretty cool. The portals were fun to play around with. Portal was definitely better, and I'm hoping that the portal gun will show up in valve's other games.