A few years ago, I read a study claiming that the larger the screen on your computer, the more productive you are. I believe it. I frequently edit one paper while having several others open, and it's convenient to have them all open simultaneously. I write my experiments in Flash, which is almost impossible to use without two screens, due to all the various floating toolbars.
This is fine if I'm in the Lab, as there I have an iMac with an attached 20 inch screen, and also another 24 inch screen I attach to my MacBook Pro. Unfortunately, I'm not always in my office. I'm sometimes at Tufts, where I am collaborating on some ERP work, and sometimes I work from home. But I don't have a bunch of extra monitors at home, and even if I did, I don't have anywhere to put them.
Enter Mimo USB monitors. This 7-inch monitors weigh less than a pound and can pack flat, so they're easy to get out of the way when you aren't using them. 7 inches is just about the right size to fit some -- though not all -- of my extraneous toolbars.
Which isn't to say it's a perfect solution. The screen is too small to host a second document window. In order to fit more, everything is annoyingly small -- and I like small font. The brightness and contrast can't be meaningfully adjusted. Those drawbacks make it annoying to use Flash or Dreamweaver ... annoying but possible. Prior to getting my Mimo, I pretty much refused to work on Flash or Dreamweaver if I couldn't do it in my office.
Of course, what I really want is a virtual monitor that could be projected onto any space (such as my glasses). In the meantime, Mimo works pretty well.