Over the past two years I have had several contacts with virtualisation both at work and at home. At work I mostly used VMware Server (free) and VMware ESX (very expensive!) to manage test environments and offer services to employees. Privately I used Parallels to get some Windows stuff going on my Mac.
Since installing Ubuntu on my work PC I’ve very happily been using Crossover Office to “emulate“ the M$ software Office 2007 (see below). That does the job very well but…. There are some things that can only be run on a native OS. So I did need some virtualisation that can make this happen. I tried VMware Server and Player but Player is too restricted and Server especially in the Beta version 2.0 is just a dog to run. It is resource hungry to the extreme. So I went to look for some other software and found the newly aquired VirtualBox from Sun. It runs on all OSes it needs to (that would be Linux and Mac and a few more) and is blazingly fast. And all that for free too!
So if you need something virtualised on your desktop look no further than VirtualBox. I think it’s the best program for the price. 
For servers and professional use there’s still no contender out there who can match VMware though. On large metal machines they are as good as it gets. Oh and so sorry gamers, there is no 3D in VirtualBox (yet?).
I think sun has a solid product here they can build on. I think in a couple of months/years they will probably try to get some of the VMware market. Also have a lookout for Parallels that is gaining ground fast in the server virtualisation world.