MobaLiveCD is a Free Portable Live CD Emulator that utilizes a Software front end for Qemu to enable you to (Run Linux on Windows). Run your Linux Live CDs and other ISO files portably from within Windows without rebooting.
VirtuaWin is a Free portable Virtual Desktop Manager for Windows. Much like virtual desktop managers for Linux/Unix, VirtuaWin enables a user to easily organize multiple applications over several virtual desktops or workspaces.
Mac On a Stick is just as the name imposes. Although for most, it isn't a complete replacement operating system, a MOS installation can include many useful applications running from an emulated Mini vMac (a Macintosh Plus emulator).
DOSBox is a portable DOS emulator or Portable DOSBox that can be used to run many old DOS games or applications that cannot be run on a modern Windows Operating System. It is great to use for those old DOS software applications or games you may have sitting around collecting dust.