You didn't say WHY you want it.
If it is for GAMING then forget it, we're not there yet. Thunderbolt is probably the first step in making that possible though.
If it's to add another monitor for basic tasks then options exist like this (note this one is for USB3):http://ca.startech.com/AV/USB-Video-Adapters/USB-3-to-HDMI-DVI-External-Video-Card-Multi-Monitor-Adapter-1920x1080
The reason external gaming devices don't exist yet is because that requires that the laptop have a high-speed connection between the graphics and CPU. In a normal computer that's the PCIe bus. There's simply not enough bandwidth if you go through USB.