.This monitor requires no extra power; it takes all it needs from the 5 volts of your USB port; and its great for laptop use because of its really portable size. This monitor can function has a display for any document that you are working form, as well to display any application that doesn't need high resolutions (it only have a 800x480 resolution display & a contrast ratio of 400:1) like an instant messenger client, your twitter page or just if you want to use it as a digital picture frame. The best part is the price: it under $100 bucks in mayor online retailers.
ThinkGeek has it for $99.99+shipping; Amazon
form $99.95+shipping and looking on eBay (something that I don't recommend) I found one for $115.95 with "free shipping" and another for a starting bid of $75.00 (it looked fake, though). Anyway, this is a good solution for your display needs and if you want to find out more about it, here is the manufacturer web site and also a good review done by the guys a PC Perspective.
Update: It works on Macs and PCs; but beware, maybe you'll need to download the drivers for you Snow leopard Mac OSX machine. Video after the break.
Here is a nice review video from the guys at The Tech Buzz.