This is a pretty general question, but it seems everyone has a different opinion and reason for their opinion. Parallels or Fusion? What do you use and why?
I've gone through the trials of both, and been very happy with the performance of each. Both show great integration with OSX, and reliability (I'm mostly using it for checking websites and apps in Windows-based web browsers). If I want to game, I restart into Windows.
Indeed, VirtualBox FTW. A bit slower in some cases (where it has to fall back to QEmu), but the command-line toolkit is impressive... and there's even a FUSE plugin for the VMDKs! It has built-in RDP, full scriptability, and lots of community-based goodies available.