...from a list subscriber:

VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use.

Presently, VirtualBox runs on Windows, Linux, Macintosh and OpenSolaris hosts and supports a large number of guest operating systems including but not limited to Windows (NT 4.0, 2000, XP, Server 2003, Vista), DOS/Windows 3.x, Linux (2.4 and 2.6), and OpenBSD.

http://www.virtualbox.org/

Related to the aobve, which actually what pointed me to it:

Virtualization continues to be a big topic among Mac users with Intel-powered Macs. Products from Parallels and VMware allow you to easily run many different operating systems on your Mac, each within its own protected virtual environment. Recently, a third player entered the market -- Sun, with its VirtualBox product.

http://www.macworld.com/article/134584/2008/07/mwvodcast59.html

Usual disclaimer: just read about it for the first time, haven't yet tried it.

I downloaded the installer and here's the "caution" given during the install process:

Sun xVM VirtualBox for Mac OS X Welcome to Sun xVM VirtualBox 1.6.2 for Mac OS X! This installer will guide you through the installation process. In a minute from now, you will be able to execute virtual machines running different operating systems on your desktop. You will find that VirtualBox delivers a great feature set and excellent performance. As one of the strengths of VirtualBox is user friendliness and excellent operating system integration, we are looking forward to receiving feedback, especially suggestions on how to make VirtualBox the most user friendly OS X application. Send your feedback to info@virtualbox.org Currently, we are aware of the following restrictions: • No support for Host Interface Networking • No support for Internal Networking • No support for audio input • No support for VT-x/AMD-V (rarely required) • No support for raw disk access • The numlock emulation isn't implemented yet • The VirtualBox kernel extension is currently accessible from all user accounts Note that we are planning to address all known issues. Should you experience a crash in any of the VirtualBox applications (VirtualBox, VirtualBoxVM, VBoxSVC, VBoxHeadless, VBoxManage), please include the CrashReporter log in the report if possible. You can find the CrashReporter Logs in: {user-dir}/Library/Logs/CrashReporter/

Wayne