TeamPlayer

TeamPlayer is a free application for Windows XP and Vista that allows you to use more than one mouse and keyboard at once when controlling your computer.

You don't have to configure anything, just install TeamPlayer and plug as many mouses as you want, it will manage them. You'll see how more cursors appear on the screen, each one in a different color.

It can be very useful if you are working with a colleague and using the same computer, by that way, you'll be able to use the computer without having to share the mouse.