I'm actually playing Half Life right now. I got a new computer recently, and I used it to finally play Half Life 2 (which was brilliant), and I realized I never beat the first one, so I decided to go back and take another whack at it.
Oddly enough, even though I'm running it off the Game of the Year CD from around 1998 or 99 on a Windows 7 64-bit machine, I'm not having a single compatibility issue with it.