I wanted to run some virtualisation software on one of my Debian boxes. We use VMs heavily at work, so I investigated my choices:

• VMWare: Free (I’m not a licence-zealot, but I don’t have to pay). Big ‘appliance’ download directory. Runs Windows on *nix, and vice-versa (I wanted to run one Windows VM). Speed might be an issue?
• Xen: Free, at least for my usage. Some use at work, so an amount of familiarity. Fast, I’m told. Doesn’t support Windows on *nix (without CPU support that I might not have).
• Virtuozzo: Expensive (but knowing people at Parallels means I wouldn’t pay anyway). Fast. Nice Plesk integration if I needed the control-panel. Doesn’t do Windows on *nix or vice-versa, though.

I know there’s a few others out there like OpenVZ (open-source Virtuozzo), vserver (which we also use at work), but I settled on VMWare in the end.

Familiarity with the desktop product, support of Win on *nix and the fact that the server provider I’m with offer an install image with VMWare on make it a sound choice for me.