Firefox (Windows, Linux, Mac): Snapper is a new Firefox extensions that can capture web pages as images. After installing Snapper, you’ll get a fish icon in your status bar. Clicking on this icon causes the fish to open it’s mouth! Now you are in capture mode. Click and drag around the portion that you want to save as image and a file save dialog pops up asking you the location to save the image.
Snapper makes it too simple. Watch this screencast.