r/mechanicalkeyboards (r/mk, rmk) - A community subreddit on Reddit dedicated to mechanical keyboards.
r/mechmarket (r/mm, rmm, mm) - A community on Reddit dedicated to the buying, selling, and trading of keyboard-related items.
geekhack (GH) - One of the most popular forums for mechanical keyboard enthusiasts.
Deskthority - Forum with a primary focus on vintage mechanical keyboards and documentation of keyboard information.
Deskthority Wiki - A wiki containing research on especially older keyboards and switches, useful for identifying a vintage board and/or its switches.
Keebtalk - A Discourse-based forum created by Huey Hoang (Manofinterests), Andrew Lekashman (Input Club CEO), Ryan Norbauer, Olivia Briggs, and Nathan Kim after geekhack was acquired by Drop.