I use VMWare ESXi 4.0 at work when I set up new system-images or production-VMs. Which works reasonably well, but hogs a lot of resources for various tasks, even on the HP ProLiant blade-server at work :s
But I'm quite pleased about my new server-VM setup @ home :D
On my Ubuntu desktop @ home, I now mainly use Virtualbox OSE (Open-Source Edition). Simple, yet powerful and quite configurable ;D
Virtualbox was easy enough to set up, and installing Ubuntu Server on a virtual machine was fun :p taking snapshots as I went along configuring and restoring web-files and what-not. I'm really content with it :) I think it emulates very well with limited resources.
Although Oracle bought Sun Microsystems Inc., it's open-source products haven't shown any slow-downs in their progress and development, except for OpenOffice that is... But the LibreOffice project saved it from corrupting it's relationship with it's open-source community.