I use VMWare Fusion. right now I only have 1 VM on my internal drive, Windows 7 x64 build 7000.
On my external drives, however I can easily pull up Windows 98, XP, and Vista, not to mention Mac OS X 10.4 Tiger and Ubuntu linux.
VMWare doesn't like it when I run more than one at once, but I rarely ever do that. I tried Parallels, but I found it a little hard to navigate, though I haven't tried any version since 2.
Equipment:
Custom PC, MacBook Pro (MBPr), 3DSxl Blue, 3DS Midnight Purple, iPhone 4S Green