If you use sites like Facebook, MySpace, Digg, Gmail, Twitter you'll want Shareaholic. Winner of the Extend Firefox contest, Shareaholic enables you to quickly, and very easily share, bookmark, and e-mail web pages via a wide array of your favorite web 2.0 social networking & bookmarking sites -- such as Digg, Facebook, Gmail, MySpace, StumbleUpon, and many many others. This is the ultimate add-on for the link sharing junkie.
Shareaholic currently supports posting to: bit.ly, connotea, digg, delicious, facebook, foxiewire, friendfeed, google bookmarks, google reader, gmail, healthranker, kaboodle, magnolia, meneame, mixx, myspace, plurk, reddit, simpy, soup.io, stumbleupon, streakr, tipjoy, truemors, tumblr, twine, twitter, weheartit, yahoo! buzz, ycombinator, bzzster, etc.
Shareaholic also lets you quickly evaluate web page popularity by telling you the number of times the web page you're on has been dugg or saved to del.icio.us.
It will show you if the item is dugg, how many diggs, and number of comments, clicking on it will either submit a new item (if it is new) or bring you to the page that already exists.
Works with both Windows and Mac.