Digg

Digg is a web 2.0 application with the sole purpose of tracking and suggesting items that are popular on the web. In terms of usability, it's beautifully simple, allowing users to nominate articles and stories, which other users can "digg". Articles with high amounts of diggs are displayed prominently on the site. The site also allows you to follow other member's diggs and comment on articles.
On of the core strategies that Digg seems to be using is interoperability and open-standards. They recently released an update allowing Facebook users to log in and use the service immediately, a move which increased new user registration by 20-30%. They are currently working to do the same for Google and Yahoo accounts. This is an ambitious move, but also a highly logical one. While all these services offer the ability to share stories, they do not socially rank them in any way. Therefore, Digg has something different to offer these services. And by making the service exceptionally easy to access for anyone who uses any of the major social networking API's, they dramatically increase their appeal.
Peter Cashmore has written a really interesting editorial on Digg, where he talks about the rise of content curation. Because new information is being created at such a massive rate, sites like Digg help users to find content they are actually interested in, and filter out the rest.

http://www.cnn.com/2010/TECH/03/18/cashmore.digg/index.html