Also if you use Windoze, don't forget the Windows Snipping tool. I keep it pinned on my Start menu -- it has additional features like allowing a delay before you do a snip, so you can do thing like bring up menus before screenshotting them. For screenshotting PDFs I recommend FoxitReader as a free alternative to Adobe. It launches way faster and has a screenshotting tool that allows you to capture more than a single screenful of the document (zoom out, then select, then zoom back in before shooting).
What I do like about the new Firefox screenshot tool is that it understands HTML elements, so it automatically highlights things like entire text blocks, even though it also lets you drag across an arbitrary screen area.