Reddit (http://reddit.com/) is a stupid site which began in 2005 (11 years ago!) with pseudonymous registration. People submit links, which can be voted up or down. The newest posts with many up-votes are shown first, then older posts with many upvotes, then recent posts with some upvotes... while the oldest and "worst" links, with the most downvotes, compete for last place. Comments are be posted in a threaded mode and can be voted up and down themselves. The sum of all a user's up and downvotes are shown on their profile, and can generally give an idea of that person's reputation on the site.
In the same niche as it, in social news, are digg.com, SlashDot, fark.com, HackerNews, and EightChanX.
When the site was just a small news site, like HackerNews, it wasn't so bad. Then it got too big and the community became too fractured, and the voting system that was supposed to work to promote good content got gamed.