Aviary, well known for providing high quality free services such as photo editing, sound editing and many more. Their services are through online only, so one has to have internet to access their free services.
Screen Capture, is another service from Aviary. The name tells you everything. Screen Capture is a free Firefox extension offered by Aviary which can take snapshots of any webpage. There are many options, like capturing the whole webpage, a region, visible portion.
After capturing the snap you can either save it onto your desktop or just upload it to Aviary for more photo editing.
It is perhaps the smallest screen capturing software and my favorite. But it does have some disadvantages. It can capture nothing out of your browser. You have to use it only for capturing webpages and that too in Firefox only.