Wikis.

A wiki is a collection of web pages that visitors can edit, modify or expand. In this way, the content grows quickly with the contributions of the people working on it.

Adding new content to a wiki page is quite easy.

1. Look for an Edit button and click on it.
2. Make changes to the existing content or write a new contribution. 
3. Look for a Save button and click on it to save the changes. 

Wikipedia, which was created in January 2001, is a universal encyclopaedia with no restrictions on use, modification or redistribution. Anyone can contribute their knowledge to the project.