Wikipedia - Wikipedia is a free online encyclopedia, created and edited by volunteers around the world and hosted by the Wikimedia Foundation.
Golden - Mapping all human knowledge
Encyclopædia Britannica - Explore the fact-checked online encyclopedia from Encyclopaedia Britannica with hundreds of thousands of objective articles, biographies, videos, and images from experts.
Sutle - Save and organize resources into a directed learning path.
Conzept - Conzept is an attempt to create an encyclopedia for the 21st century. A modern topic-exploration tool based on Wikipedia, Wikidata, Open Library and many other information sources.
QuestBook - The fastest way to teach and learn