Leftypedia
Encyclopedia of subjects
Omniglot
A guide to writing systems and languages, with useful phrases, tips on learning languages, multilingual texts, and much more.
RP Photonics Encyclopedia
The open-access encyclopedia for topics in laser physics, laser technology, nonlinear optics, fiber optics, quantum optics, and general optics. Very useful particularly for the photonics industry and for researchers!
Scholarpedia
Stanford Encyclopedia of Philosophy
The Encyclopedia of Earth
The World Factbook - CIA
Webis - Uni Hamburg
Wikispooks