I use Awesome Screenshot http://awesomescreenshot.com/ and find it very easy to use and there are extensions for Chrome, Safari and Firefox. Basiically you can capture a whole page or just a visible part, annotate with arrows, rectangles circles and text etc and then either save the new image or email out a url. It is developed by the team at Diigo the social bookmarking app and is free!
I use 'gimp', which is freeware available for Windows, Mac or Linux. Here you have an 'acquire' option with parameters to delay the capture, whole screen, selected window, etc. Once captured, it will display the image and you then have the full suite of editing tools (very similar to Photoshop) and filters. A little tricky at first to master, but brilliant once you are used to it...and free. Widely used in the Linux world.
FS Capture is really simple
I do think it is worth paying for one of these products, to have the advantage of the managed upgrades that come along quite frequently.
I use OneNote which is part of the Microsoft Office suite and is an electronic scrapbook that's amazingly simple to use for handling all kinds of notes/scraps/snippets/images etc. If you have MS Office on your PC, you probably already have it. If you have, simply press the Windows flag key and S at the same time. The screen turns a little opaque, you use your mouse to highlight the area of the screen you want to capture and voila! The image appears as a page in your OneNote book and the site from which you captured it plus the date is supplied next to it for reference too. I used to use SnagIt but now find it easier to use this since I don't have to fire up another application - I just grab what part of the screen I want when I want.
Learning something new every day - thanks for that tip, Dave...and I agree that OneNoteis great too!