Sun has just released an update version to their free, open source desktop virtualization client, VirtualBox. They are saying the major feature is that they now provide support for SMP (multiple CPUs) for their virtual guests! In addition to that, they have improved 3D support (not that it was too shabby before).
Also, remember that this is available for download on all major OSes (Windows, Mac OS X, Linux, and even Solaris).
For being a free virtualizer, VirtualBox is definitely some good competition for products like VMware Workstation/Fusion and Parallels. It doesn’t seem to support 64-bit guests yet, though.
Check out the full Changelog and try it out! Let us know what you think!
Edit: stu pointed out that 64-bit guests are in fact supported, but you have to have VT/AMD-V support on your CPU. Most modern systems have this these days, however the desktop I used when I tried it on did not. This was my mistake for assuming it wasn’t supported.

64 bit guests have been available for a while http://www.virtualbox.org/manual/UserManual.html#intro-64bitguests
Oh, My mistake. When I tried it in the last version (2.2.4) I had trouble… however this is because the desktop I tried it on does not have VT/AMD-V support on the CPU (An original Athlon64 X2, socket 939). I never thought to try it on another machine and just assumed it wasn’t supported.
Thank you for pointing that out! I will edit the main post to clarify.