At least once a week, until the end of race (NASCAR) season. "Raceview" only works in Windows, for now (live, 3D car/track representation).
Additionally, amateur radio logging software, using serial communications with radio over USB.
Both applications through Win2K running on Parallels.
I haven't used it in a while, but I also have XP installed in a Boot Camp partition to run R/C aircraft simulator (RealFlight 3.5). I might run in Parallels, now that it has some 3D support, but RealFlight is very graphics intensive. Runs great on 2.33 MBP booted natively, but I'd probably have to cripple it somewhat to run reasonably in Parallels.