VMware
VMWare is an emulation suite for Intel x86-compatible architectures which allows a computer to run multiple instances of operating systems simultaneously, including (but not limited to) Windows, Linux, and BSD variants.
It is similar to its counterpart Virtual PC, however VMware has architectural differences that affect the way software interacts with the hardware. The net effect is that some operating systems under VMware may interact better with VMware's emulated hardware, and some will not.
With VMware you can try LiveCDs without burning them in a CD burner or rebooting the computer.
See also : User Mode Linux.
