As I understand it, Pligg is an open source CMS designed to provide your site with Digg
Looks quite interesting and while still in beta, sure seems to be getting some notice.
Pligg's Web 2.0 user interface gives your visitors a reason to come back to your site by making them decide on the site's content and giving them the chance to social network. By giving users the ability to collaboratively submit and dismiss articles, you are offering an experience that users are happy to take part in on a daily basis.
It's donationware - consider making a donation to support the project if you like what they are doing!