Aviary has just launched Screen Capture Talon that can let you capture any webpage from any browser, by simply putting Aviary.com/ in front of the URL. You can capture a page at different resolutions or to only capture a portion of the page as well.
After capturing the page, you can edit it, add simple markup to it and save it to your desktop (or host it online with a single click). It works on almost any public page on the Internet, with any browser. It can even capture Flash. However, it does not support multiple browsers / operating systems at the moment.