What is VMware?
-Lets you to run another Operating System when your current Operating System is running.

Where to get it?
- http://www.vmware.com/products/ws/

Why to use VMware?
-Simply it is very useful for testing purposes, you may test your software’s compatibility on other OSes. Also you may
actually want to test an OS itself before considering to install it to your machine.

How to get an Image?
-You may install an OS by hand on VMware and then you can make your own image or you may get an existing image from somewhere.
(Check below links.)

How many available images?
-There are many, Windows 95/98/2000/XP/Vista , Linux , SOLARIS , BSD based Oses.

Does VMware Support all OSes?
-Currently it is supporting all existing OSes.

Where Can I download images?
-Check the below links, i put some OS image torrents.

I have tried it myself, found it really useful, I have installed SOLARIS , Debian Linux to Windows XP, results are great, performance can be considered as good. Actually it depends how much memory and hard disk space that you allocated for the OS, typically 256 and more will perform better, also hard disk space depends which application(s) you will install to the OS.

VMware Image Downloads
Debian Sarge (115 MB)
Fedora Core 4 (283 MB)
Fedora Core 5 (976 MB)
Fedora Core 6 (873 MB)
Fedora Core 7 (890 MB)
Fedora Core 7 (898 MB) newer version!
FreeBSD 6.0 (129 MB)
FreeBSD 6.1 (172 MB)
FreeBSD 6.2 (235 MB)
Ubuntu Server 6.06 (172 MB)
Ubuntu Server 6.10 (171 MB)

Some other posts that you may like :