thumbalizr

From the site:

With thumbalizr you can take screenshots of any webpage you want.
Use it for your presentations, documentations, visualisations or your webpage.

Just enter the target-URL and press the "thumb it"-button. Wait a moment and a screenshot of the desired page will be taken. You can choose between a screenshot with a fixed height of the screen or the complete webpage.

That's it! Just click one of the resolution-links (320, 640, 800, 1024 or 1280 pixel width) to download your screenshot.