To start with, when users head to one of Digg’s outbound links, the toolbar will let them see how many times the story has been dug. This remains true even if the story does not appear on the front page, which can often take an overwhelming amount of diggs to accomplish. It will also allow users to digg the page themselves and includes options to see any related stories that have been featured on the site or share a particular story with others via a Facebook note, an e-mail, a Digg shout, or a Twitter message.
None of the above requires any software installation whatsoever, with the toolbar actually operating as a kind of overlay that embeds the pages beneath it. Digg has not released information on when it'll be available on a wide basis, though I'd hope soon as it looks very interesting.