I use an older MacBook Pro as my “daily driver”, and like most Mac laptops it only supports connecting one external VGA/HDMI/DVI display (depending on what adapter you have).
Newer MacBook Pros that have Thunderbolt support connecting up to two displays per Thunderbolt port, but mine predates this technology. Even if I did have Thunderbolt, Apple is the only one currently making Thunderbolt displays, and they run about a grand each.
I currently have a 19″ external display connected to my MacBook Pro, and wanted to add another. New 19″ displays can be picked up for $99 or less these days, so I set out to see if there was an economical way to get more screen real estate.
What I found surprised me. There’s a company named DisplayLink that appears to have set the standard for getting video out of USB adapters, and they have sold this standard to a wide variety of third-party companies that are making USB to VGA/HDMI/DVI adapters. These companies include: Kensington, StarTech, Plugable, Targus, SIIG, Accell, HP, Lenovo, HIS, Diamond, IOGEAR, SWELL, Winstars, and Ationa.
If you go to DisplayLink’s website, you can download free Mac drivers that support OS X 10.6-10.8. Once the driver is installed, you can connect any DisplayLink compatible third-party USB display adapter to the Mac, and it shows up in the Displays system preference pane and works along side other displays. Up to 4 monitors can be connected over USB. This of course could also be used on desktop Mac models like the iMac or Mac mini, and Windows DisplayLink drivers are also free to download.
These DisplayLink compatible USB adapters are affordable too. Many run in the $50-$75 range, and I picked up a SIGG USB/VGA one from eBay for $30 shipped (search eBay for “displaylink usb” without the quotes).
Keep in mind that adding additional displays using DisplayLink compatible USB adapters is great for day-to-day work, but these USB driven displays don’t have a graphics processor powering them. From my experience I notice no difference using my display connected via USB, but it would not work well for gaming. The only glitch I’ve found is that I cannot take screenshots on my USB display.