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