Greenshot is a free (under GNU General Purpose Licence) tool, which offers more than just the standard Print Screen button would do on a PC.

If you’ve ever wanted to capture just a small part (region) of the screen, then this is a great tool for doing that. It allows you to map itself to your Print Screen button, meaning you can still have the convenience of pressing that button. If you do, after pressing it you’ll be presented with a cross-hair to select what you want to capture on the screen. After letting go, it will then let you decide whether to save it straight away (as a jpg for example), or whether to launch it in MSPaint.

You can find more information, and a free download, here: