It took me a while to find a screenshot tool as useful as the built in screenshotting tool in OS X. I looked again today and found the import tool that comes as part of ImageMagick.
$ import screenshot.png
You can use it to capture an area on the screen with the above command or you can capture a whole window.
$ import -window root screenshot.png
I will probably throw this into a script and bind it to a key for ease of use.