I am currently trying out Virtual Box. It's an OSS virtualization product managed by Sun.
I come from having used a mac these last few years and had gotten used to the warring between Parallels Desktop and VMWare Fusion. Both of those progressed nicely over time and remind me why competition is so great for the consumer.
Virtual Box takes a different approach with its open-source codebase, with the option to utilize some proprietary extensions in the closed-source version.
My plan is to use it as a container for a winxp machine for testing and windows-necessary stuff like MS Office. Currently the host for that vm is winxp too, but when the vm is ready, I'm planning to move to Ubuntu (jaunty jackalope) as the host.
So far, Virtual Box has satisfied my needs. It has a seamless mode to sort of have the programs running in vm look like they are just programs running in the host. It's not as elegantly done as the other two I've used in the past, but it's functional. For example alt-tab, when in seamless mode, only switches between the vm's applications. The Virtual Box Guest Additions provide the mouse pass-through mode so you can more easily switch between host and vm. And from what it looks like networking and audio also appear to work (in winxp and ubuntu at least).
Things I wish it had were drag-and-drop file support and a better seamless mode. One other thing I found out from the IRC channel is that in order to move a vm from one platform to another, you need to make sure you export and import the vm. I was used to just copying the vm file in vmware fusion at least, but that's not a frequent operation. Features/enhancements like those may come in the future though and it's hard to argue with the price.
One thing that is quite nice about virtual box though is the community. If I have a question, I search online or I can check out the IRC channel (#virtualbox on irc.freenode.net). I had some questions earlier and they were quite friendly and responsive (on the IRC channel).
All-in-all, I am quite impressed with it thus far.