Just about everybody by now knows about VMware's Player. If you don't, then you should. The player is free as in beer, and there are pre-built "appliances" where complete Linux distributions are already installed and ready-to-run with the player. All you have to do is snag one and have the player run it. The system I have all this installed on is altair 4 at work, the Boxx system with the aging Athlon FX-55 and 4GB of DRAM running Windows Server 2003.
So far, using VMware Player version 2, I've downloaded and tried six different disribution appliances: Debian Etch, Fedora Core 7, Fedora Core 6, Suse 10.2, PC-BSD 1.3.0, and RHEL 4 Update 4. What follows are a trio of screen shots with the player running an FC7 distribution.
This shot (above) is a screen shot produced by Fedora's own screen capture (via Gnome). This shows the complete desktop as it would appear if my monitor were capable of completely displaying 1900 x 1440. This is the first time I've started this virtual machine, and it has found and is downloading updates.
This shot shows the Windows Server 2003 desktop with the same Fedora 7 desktop running in VMware.
Here's FC-7 again, but I've maximized the player. There's a scroll bar across the bottom of the screen that allows me to see the entire FC-7 desktop if I want. Right after taking this screen shot I changed FC-7's screen resolution so it would fit in the regular physical limitation of the Samsung LCD (1280 x 1024).
So what's it all good for? It's a quick and very easy way to sample distributions and run simple tests to see what works and what doesn't without the installation pain. It isn't perfect and a virtualized distribution will never be as fast as one running natively on the hardware, but for a lot of things, it's Good Enough.