I quite often take a screenshot - generally because I want to retain accurately a view of how something appeared on my screen, sometimes because the context of screen presentation is more interesting than a bare photograph.
Typically - at home at least - I'll grab a screenshot with Picasa running in the background, because it has a rather neat capability whereby it detects the action, imports the image into the 'Screenshots' folder and, if I click the little sidebar that temporarily pops up as it does it, makes it available to edit. Extremely slick.
But... sometimes I want to annotate it - arrows, boxes, bits of text - and Picasa is not particularly good on that front. So I Googled around for something that is - and there's a surprising paucity of simple software to do just that. Eventually, however, I stumbled upon Screenpresso, which is a) Really capable and b) Free, unless you want to do esoteric things with HD video.
Screenpresso does rather more than I immediately needed - and playing around with it, I discovered a feature that's really rather clever: It can grab a screen shot of a scrolling web page and present it as a single image - shown here with the Guardian website. I'm not sure this has many uses outside the web development arena, but it's a fascinating perspective on the websites you use every day. You can grab the full resolution image here if you're interested.