Reddit (stylized in its logo as reddit) is an American social news aggregation, web content rating, and discussion website. Registered members submit content to the site such as links, text posts, and images, which are then voted up or down by other members.
Quora.com is a question/answer service allowing users to pose questions that other users can answer, each question and answer can have comments, each users can write one individual answer and there can be a summary answer.
4chan is a simple image-based bulletin board where anyone can post comments and share images. There are boards dedicated to a variety of topics, from Japanese animation and culture to videogames, music, and photography.Warning
4Chan is less moderated compared to other social networks and could be offensive to some people
Uncensored free speech alternative to the extreme Reddit and Voat forums, democratically civilized by Graham's Debate Pyramid, no illegal transactions nor porn. Truth-seeking anti-corruption news are among the popular themes. Android app, iOS soon.
Unsatisfied with the few link sharing communities that were available and their proprietary, untransparent and non-secure nature, ziq made the original raddle.me (then known as raddit.me) with Wordpress, but it had a lot of limitations.
News for nerds, stuff that matters. Slashdot (aka /.) is a technology-related news and discussion site with a focus on Linux, free and open source software and information freedom and privacy issues. The software it runs on, called Slashcode, is open source.