Be sure to check out the respective creators' sites too!
Knowledge bases
The World Atlas of Language Structures (WALS) Online, a large database of structural (phonological, grammatical, lexical) properties of languages gathered from descriptive materials (such as reference grammars) by a team of 55 authors.
Mark Rosenfelder's conlanging and conworlding work probably does not need to be introduced to conlangers, but his articles and book reviews are no less interesting: an extremely interesting one about conworlding is his article on cities, reviewing Jane Jacobs' work.