I noticed a widget on Jeff Atwood's website that displays his ranking on all sites on the Stack Exchange network.
Where can I get this widget from?
Go to your user profile (click your username in the header). The very last line in the left hand column says "Got Flair?". Click that link. On that page, there are two textboxes at the very bottom containing HTML code. The first one is site-specific for the site you're viewing; the second one is across all Stack Exchange sites, which is the one Jeff has.
You can also right click on any of the flair at the top of the screen to download it; however, if you use the URL that is provided in the HTML code, the flair does autoupdate.