I personally work with two monitors and it's nice, lots of room to put everything.
This program does this across multiple computers and the computers can be any combo of Mac, Windows, Unix.
It's not a monitor switch or anything. You move the mouse from one monitor to another even sharing the same clipboard.
I only have one computer these days so I can't try it out but it looks pretty kewl.
Synergy
http://synergy2.sourceforge.net/about.html