Teleport is really handy. It doesn't let you do everything another monitor would do, but you can do a lot with mounting shared volumes so you can open documents in one whatever monitor and the clipboard syncing makes it almost transparent. The main thing I use it for is reading documentation-pretty much the same as I used my PowerBook at home before teleport but I don't have to move my hands to get a new page.
And the price can't be beaten.