Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
QEMU is an open source cross-platform emulator for Linux hosts. It allows you to emulate a number of hardware architectures (x86, x86-64, and PowerPC are currently known to work, with others, including SPARC and MIPS, in development). QEMU thereby lets you run another operating system on top of your existing OS. Going through the process of installing and configuring QEMU not only gave me a worthwhile new software tool, but also helped me learn a few things about Linux.
Right from the beginning, I was impressed by the quality of QEMU, as well as the feature set.
There are two ways to install QEMU: either in binary format, for Linux i386 only (a tar.gz file that you simply unpack into your top-level directory) or from source.