MobaLiveCD is a Free Portable Live CD Emulator that utilizes a software front end for Qemu to enable you to Run Linux on Windows. It allows you to boot and run Linux Live CDs and other bootable ISO files directly from within Windows without rebooting your system. Even better, the entire setup can be stored and launched from a portable USB flash drive, making it ideal for testing, diagnostics, and demonstrations on the go.
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.
Jnes is an old school Nintendo Entertainment System (NES) emulator that can be run from a portable device such as a USB flash drive. This lightweight NES emulation software not only provides accurate gameplay but also includes an advanced database of Pro Action Replay and Game Genie cheat codes to enhance your gaming experience. Best of all, it supports Kaillera netplay so you can play with or against your friends across the network in real time.
Mac On a Stick aka: Mini vMac, just as the name imposes, is a portable Macintosh emulated operating system that can be booted and run from a USB flash drive. 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).