When we last left this topic I had just installed Ubuntu, but in reality you can run virtual operating systems on just about any OS. I choose to install the latest 64-bit version of Ubuntu, and for virtulization Oracle VirtualBox (formerly Sun VirtualBox before Sun was acquired by Oracle). While I could have used VirtualBox on Windows 7, I just do not feel comfortable relying so heavily on a Microsoft OS, especially before Service Pack 1 
Anyway…as I said, I installed VirtualBox (here a good tutorial for Ubuntu), then installed my client OSs. The first client was Windows XP, then Ubuntu 32-bit, OpenSolaris, FreeBSD, and finally I created a “catch-all” virtual machine. The catch-all has two virtual hard drives and triple-boots DOS 6.21, Windows 3.1, and Windows 2000. DOS is there because it is still a widely used OS for single purpose embedded devices like cash registers. Windows 3.1 is there for nostalgia because it was my first version of Windows, and Windows 2000 is there mainly to make it easy to update the other two.
Here is a screen shot of my desktop:
The screenshot above is from my dual wide-screen monitor setup. From right to left: Windows XP in fullscreen mode running the free Visual C++ 2008 Express Edition, DOS 6.21 (you youngsters may have never seen MS-DOS Shell), then finally OpenSolaris.
Later I plan to set up FreeDOS, Haiku (inspired by BeOS), GNU HURD, other BSDs including PureDarwin (Darwin is the core of Mac OS X), and other Linux distros. Testing out new OSs or distros has never been easier! Now on to some serious cross-platform programming…..