Some of my favorites:
/r/LifeOfNorman – A subreddit about writing small, fictional tales about a fictional character named Norman; a rather unremarkable fellow
/r/UnresolvedMysteries – One of my favorite subs about mysterious cases that have yet to be solved
/r/AMADisasters – A sort of museum for AMAs that have gone totally wrong
/r/AteThePasta – When people fall for copypasta in the wild.
/r/DepthHub – A bit like /r/bestof for for really in-depth discussions.
/r/redditwritesseinfeld – A fun little sub where people write scenes from Seinfeld based on prompts
/r/TheGrittyPast – a great little subreddit about accounts from history not often talked about because of their tragic and often violent nature.
/r/HistoryAnecdotes – Pretty much speaks for itself, mostly content from the creator of the sub who also does TGP, but others contribute too
/r/FanTheories – A great sub full of some pretty great theories about movies, shows, and books