* https://reddit.com/r/AskHistorians - All things history. Quality of discussions are way better than on most other subreddits. Highly moderated

* https://discuss.bootstrapped.fm - bootstrapped businesses

* https://lobste.rs - kinda like HN but for tech only

* https://slatestarcodex.com - it is more blog than a community, but great nevertheless

I used to frequent other communties but these are the ones that stood the test of time