Ncomputing sells a product called the X300 which will allow you to run 3 additional workstations off from one PC. It uses a PCI card in the host PC and 3 small boxes where you can plug in keyboard, monitor, etc.

What you do is add a PCI card to the host PC and connected it to the workstation boxes. These boxes are very small (about 4"w x 2 1/2"d). You could add up to 3 PCI cards to a host PC for a total of 10 workstations (1 PC + 9 workstations) Of course your host PC would need to be very powerful to run 9 of these units, but if you are running 3 boxes, their requirements are very reasonable. My workplace is currently demoing with one card and 3 X300 boxes. It has worked out very well so far. I will past a couple useful links below. Hope this helps.

http://ncomputing.com/ncomputing/download/flyer/X300_flyer_(eng)2.pdf
http://ncomputing.com/ncomputing/products/direct.php