General
World Atlas of Language Structures (WALS): a database of structural properties of languages around the world. Includes a visualiser for where certain properties tend to occur.
Simon Roper (YouTube): videos on linguistics, especially historical linguistics and Germanic languages, as well as some philosophy and anthropology.
Dr Geoff Lindsey (YouTube): mostly phonology and (recent) sound change, and some sociolinguistics.
Documentation
The Lingua Descriptive Studies Questionnaire: a language documentation questionnaire. I use it for conlanging.
Phonology
Speech Accent Archive: recordings and phonetic transcriptions of the same English passage recited by speakers of different linguistic backgrounds.
Searchable Index Diachronica: an index of documented sound changes across languages. There are a few issues with this, so it might be kind of iffy as a historical linguistics resource, but it's still a very good guide/inspiration if you're conlanging or learning about sound change in general.
TriSCA: a sound change applier.
Orthography
How to Create a Script: a beginner's resource for making conscripts, and how orthography works in general.
The World's Writing Systems: basic information (and links to further information) on every known writing system.
Semantics
Database of Cross-Linguistic Colexifications
Morphosyntax
Notes on Language Creation - Ergativity
Conlanging
Awkwords: a word generator (now hosted on a different site).
Monke: another word generator.
The Smiley Award: a (discontinued) award given to cool conlangs.
