Ratoc Systems International, a to us unfamiliar company has launched the REX-USBDVI external graphics card. Although it seems like it's only available in Japan at the moment, it's likely that the company will launch it in other markets in de time. Ratoc seems to specialize in PDA graphics solutions and various notebook upgrades.
The REX-USBDVI supports resolutions of up to 1,600x1,200 or 1,680x1,050. It's not going to be powerful enough for playing games with, but you can attach multiple units to your PC or notebook for some multi display action. This isn't the first such device, but it's the first one we've seen with a DVI connector.
At US$130 it might be cheaper to add a second graphics card, even a PCI or PCI Express x1 card. However, if you're looking for a multi display solution for your notebook, this looks like a decent option. Another downer is that the REX-USBDVI isn't fast enough for the new Aero interface in Vista, although it is Vista compatible.