I’ve created a new category called Encyclopedias on my English Themes for Beginners and Early Intermediate page. Links to several online encyclopedias appropriate for English Language Learners can now be found there, though I’d like to highlight the one listed first — the Simple English Wikipedia.
I know there are strong opinions about the use of Wikipedia on both sides of the issue. I’ve found it helpful and accurate. My students have had the same experience with this simple English version, which is geared towards English Language Learners.