Tip: If you have a laptop with few available USB ports consider adding an external monitor that has USB ports built in (USB hub). Take for example my Macbook Air. It has only 1 USB port (bummer) but when I hook it up to my Dell Ultra Sharp 19" monitor I now have 4.
2 USB ports on the Side and 2 on the bottom. Everything runs perfectly and I get an extra monitor out of it. If you need more than 4 just add another hub to the USB upstream port.