What are the best virtualization apps in osx?
I mainly use a VM for linux (any distro) and windows server 2008 and 2003
I use Parallels 6 and it's been excellent so far. The only OSs I've tried on it are Ubuntu 9.x, 10.x, 11.x; Windows XP and 7; and all of them ran great. Make sure you have RAM to spare though
There's also VMware Fusion but I haven't used it all that much, so I can't comment on that.
P.S.
I've also tried VirtualBox but didn't find it to be super stable.