I use VirtualBox to run a Microsoft Windows guest on my iMac. For some reason, VirtualBox did not notify me that version 3.0 had been released. This new version includes symmetric multi-processing (SMP) for guest systems, Direct3D 8 and 9 for Windows guests, and OpenGL support for Windows, Linux, and Solaris guests. The change log includes many more fixes and new features. Additionally, version 3.0.2 has been released which builds on the changes introduced in VirtualBox 3.0.
If you haven't already, try out VirtualBox and let us know what you think.