4chan is described as «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» and is a popular website in the social & communications category. There are more than 25 alternatives to 4chan, not only websites but also apps for a variety of platforms, including Self-Hosted solutions, F-Droid, Android and Mac. The best alternative is SaidIt.net, which is both free and Open Source. Other great sites and apps similar to 4chan are Aether, Tildes, Raddle and [RAMBLE].4chan alternatives are mainly Social Networks but may also be Social Newss or Imageboards. Filter by these if you want a narrower list of alternatives or looking for a specific functionality of 4chan.
