Articles and illustrations from the Encyclopaedia Britannica, 9th Edition (1875) and 10th Edition (1902). Searchable by keyword and browsable by subject index.

Displays Wikipedia articles and their simpler counterparts side by side.

Provider of learning knowledge products through its online resources and distributed books and products.

Knowledge resources featuring almanacs, atlases, encyclopedia, discitonary, thesaurus, and white pages.

User-written knowledge-base with comments sections for all articles.

Offers a mashup of Wikipedia articles, Twitter tweets, and breaking news.

Encyclopedic guide featuring articles on various topics with several accompanying photos and maps.

Thousands of cartoons by Tim Hunkin, offering simple information in many different categories including biology, music, psychology, music, language, religion and engineering.

Features a peer-reviewed open-access encyclopedia, containing knowledge that is curated by communities of experts.

Visual encyclopedia project that is a collaboration between Caltech and UCSD.

Offers a large assortment of topical wikis, as well as the traditional ability to edit them.

Free open-source multilingual encyclopedia with volunteer editors.

Official site promoting its excellence in the production of award-winning encyclopedias, reference sources, and multimedia projects for the home and schools.