I’m fooling around with Sun’s VirtualBox — virtualization software that comes in both an open source GPL version and a “free for personal use” closed source version with some more features. It seems pretty nice so far. It compares favorably with VMWare given the price. I haven’t done much with it yet, though, so it may turn out to have rough edges.
I’m running an Ubuntu 8.04 LTS box on my Mac. The Mac version looks to be the least polished, since it’s the newest. VMWare Fusion has a lot more hardware support (e.g. sound). I need to take another shot at VMWare/Ubuntu v.latest to see if the guest additions are now supported.