Just to name a few.
https://www.reddit.com/r/AskHistorians/ is a real jewel of the modern internet. It is ruthlessly and capably moderated and hence has an incredible amount of good content
/r/corgi
/r/dadswhodidnotwantpets - Pictures and videos of men (and sometimes women) cuddling with animals after initially rejecting them
/r/sbubby - Pictures of edited logos that look like other logos (e.g. Changing "The Price is Right" logo to say "The Rice is Aight")
/r/wimmelbilder - Imagine the Where's Waldo? images, but with pictures filled to the brim with everything.
There's very little censorship of discussion compared to the rest of Reddit, the only rule seems to be politeness and topicality. It's almost like HN in that sense.
/r/netsec is my daily habit too. Always bound to find detailed explanations of vulns