A quick note about forums: Forums are an awesome way to get to know people in the business who know different things than you, and to share what you know with those who know different things than you. Be kind and courteous on all forums. While there are some subreddits listed below (read the description for the Theatre Subreddit for more), there are subs for other fields, some of which are archived (like /r/directing), and are inactive. The EntWork site does not, and will not now or in the future host forums.
Control Booth is a broad forum focusing mainly on the technical and design aspects of theatre.
If you’re unfamiliar with Reddit, as Sir Alec Guiness said (as Obi-Wan Kenobi) of Mos Eisley Spaceport, “You will never see a more wretched hive of scum and villainy.” Reddit has forums (called subreddits) for Everything, and I mean it. /r/Theatre, the Theatre subreddit, is a catch-all for everything theatre. From comments on shows to monologue suggestions and directing tips, this sub has it all.
Naturally, the Actors have their own subreddit. There is a fair mix of stage and film in this subreddit with excellent resources from Equity and SAG-AFTRA performers.
A subreddit for dancers.
Of course, the technicians have their own space, too. With lots of knowledgeable people, /r/techtheatre is a great place to ask for and maybe contribute some help. There is even a weekly No Stupid Questions thread for when you have a question you think is just too stupid to ask, but you don’t know the answer to.
A subreddit for lighting designers of all stripes, including both theatre and architectural.