I've been using VirtualBox (the Open Source Edition) a fair bit recently. My desktop machine fairly hefty: an i7 860, 4 core HT and 8GB RAM. I leave it running Kubuntu and VirtualBox on top of that. Then I have a workable, if by now slightly dated, desktop and all the KDE applications that I need. In VirtualBox I run the machines I'm interested in for development. That's an OpenSolaris instance and a FreeBSD one (just resurrecting that interest now and I haven't even gotten around to checking out area51 yet). I also keep a Fedora and a Kubuntu VBox around for comparison purposes. So plenty of ways of using up the CPU power. Next to the desktop machine (also under my desk) is my spare box, which is an amd64 6250e, dual core, 6GB RAM.
The guest additions make the applications running in the VBox interact with the host machine without mouse or keyboard grabs; then the KDE application running inside GNOME in OpenSolaris is very nearly just another window on my desktop.
I use the OpenSolaris VBox to compile KDE packages. In a very simple test -- building QScintilla -- I find that the virtual machine outperforms the physical machine sitting next to it. Compile goes from 3 minutes to two, running the same software stack. I'm very much surprised, because the clock speed difference isn't that great and I'd expect there to be some non-trivial virtualization overhead.
The fact I can share a folder between the host OS and the VBoxes is quite useful, since it means I only need a single checkout of KDE SVN. I can patch from inside each virtual machine, experiment and eventually push things back out. FreeBSD doesn't support directly shared folders, but I can still network-mount everything. This then means that it's become much easier for me to test changes across OSsen. No longer will I horribly break Linux builds with a little Solaris patch (er .. I don't think I've done that before, or maybe I just don't remember).
What would be really keen is appliances: a VBox image that is an instant FreeBSD / OpenSolaris / OpenSUSE development environment. That would work for FreeBSD and OpenSUSE, I think, but the license terms of Sun Studio presumably preclude redistribution. That's unfortunate, because it makes it harder to get development done (although, come to think of it, a system which has everything installed except for the one tarball of Studio might not be encumbered). It would serve a different purpose than the SuSE build service which does builds for many many different OSsen -- desktop development in a separate environment.
One thing I intend to add to the stable is a Linux VBox with Sun Studio for Linux installed on it, so I can try some stuff related to the compiler. I recently finished up packages for KDE-PIM and there were many patches needed; there's some template magic that the Sun compiler just doesn't understand, leading to dozens of KCalCore::Incidence::Ptr() cast constructors being patched into code that's otherwise a model of tidyness. Only once I've got something sensible set up will it be possible to apply power machinery (in casu KDAB Marc, C++ template guru) to the problem. In any case, we have packages for all the parts tagged as KDE 4.5.68 (snapshot from trunk) for OSOL except for bindings, edu, kmix and plasma-addons. The problem in kde-edu is actually a bug with Boost and Sun Studio, so that's going to take a little more work to fix.