Picnik.com, one of the favorite Flash-based tool for editing images online just keeps getting better and more useful.
They have a excellent add-on for Firefox 2.0+ that will help you grab a screenshot of any webpage with a simple right click and it will automatically export the image to Picnik image editor for further editing (like resizing, including borders, overlay text, rounded corners, etc)
For instance, this page from Steve Rubel was enhanced using the Vignette / Matte effect and it's trasformed from a boring screenshot into something that immediately invites attention.
You can screen capture either the visible portion of the webpage or the entire webpage even if extends several folds. Very handy for saving snapshots of webpages (like an ebay paypment receipt) or for creating thumbnail images of your websites for MyBlogLog, Facebook or other services.
Picnik Firefox Add-on | Picnik.com
The Picnik Screen capture extension is Firefox only but here are some screen-capture options for IE. And some more Firefox extensions for taking screen capture of websites and HTML web pages.