Cameo is a neat BlackBox for Window or (Windows Shell). Basically, it is an open source portable desktop environment that can be used in place of Windows Explorer.
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.
Emerge desktop is an Open Source portable alternate replacement Windows shell. This shell works by temporarily replacing your Windows Explorer user interface through the implementation of MinGW.
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.