PrtScr is a free screen grabbing tool that comes packed with essential features.
By default it's called via the PrtSc key, for instance, but you can also access it via Ctrl+PrtSc or the hotkey of your choice, so the program won't interfere with other apps.
PrtScr can then capture the full screen, the active window, or a rectangle of your choice. There's also a freehand selection option that works very well, once you've figured out the basics (which only takes a couple of minutes). You can optionally include the mouse pointer in the grab.
And once the grab has been taken, PrtScr can scale it down, copy it to the clipboard, save it to the desktop without asking (JPG and PNG formats are supported), or display the settings dialog so you can decide what happens next.
While not quite up to the standards of the commercial competition, PrtScr is an excellent tool that makes it quick and easy to capture the images you need