First you probably can use the external monitor AND the screen on the laptop.
Thats how I run mine when in my office.
Then there is this.
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=6594328&CatId=467
Not recommending it, but just showing as an example.
A friend uses something similiar, I don't know what brand etc, but I believe he drives 4 monitors via several of this and a USB hub.