I’ve been pretty impressed with the Diamond BVU195 display adapter. I’ve found USB to display adapters almost universally suck. The BVU195 seems to be one of the exceptions to the rule though. It’s definitely no replacement for a video card with multiple display ports but I work on a Macbook Pro so I’m constrained.
The adapter ships with installation media but it’s only for Windows. The manufacturer’s product page has a drivers section and even mentions Mac OS X but the download doesn’t seem to contain any actual OS X software.
Thankfully the chipset manufacturer for the product does have an OS X driver available. The driver is currently listed as a beta but I’ve been using it with OS X 10.7 Lion without issue for several months now. You can find the DisplayLink drivers here: http://www.displaylink.com/support/mac_downloads.php