Wikipedia
Vikidia
Vikidia is an online encyclopedia wiki for children that runs on MediaWiki software. It has been available in French, Spanish, and Dutch (as WikiKids) for years, while the Italian, Russian and English versions were only created recently. The newest one is the Catalan Vikidia.
The site focuses on presenting articles suitable for primary school children from eight to thirteen years of age. Most of its users are children, though there is no age restriction. Due to its use of simple language, it may also be a useful resource for those learning other languages, though the Simple English Wikipedia, which has a similar principle, is more geared towards mature learners of English rather than younger readers.