Microsoft VirtualPC is not opensource, but is free. However, it only supports 16-bit color, and has problems with Ubuntu out of the box (you'll have to search the web for non-standard installation instructions). I've used it successfully with Debian and PCBSD.
VirtualBox from Sun is opensource and free. I have not tried it but friends who have like it.
VirtualBox from Sun is opensource and free. I have not tried it but friends who have like it.