Reddit - Social bookmark engine

Reddit is a web-based social bookmarking system. It allows multiple users to store, share and tag their favorite links online. Users will provide the content (links) and decide through voting what's good and what's junk. Links that receive community approval bubble up towards #1, so the front page is constantly in motion and filled with fresh and interesting links. Sub groups (community) could be created and each groups could be moderated by its owners.

The content are indexed and available for search. Reddit has millions of page views per day and it has highly scalable architecture, written in Python. It has API support, Private messaging, User profile management, Comments etc.