I'm amazed none of these have been mentioned yet. Here are some of my faves

Explaining stuff that's happening

• Real Life Lore (I'm almost signing for Nebula because of him, gorgeous map graphics)
• Cold Fusion
• Wendover

History stuff

• Fall of Civilizations - Hands down the best documentary-like content I know of. Takes forever for new stuff to show up (because research), but it's so fucking worth it!
• Study of Antiquity and the Middle Ages
• Kings and Generals (the narrator butchers every non-anglo name, it really grinds my gears)
• Heroes and Legends - The long form, documentary-level videos are amazing. Watch the ones on Gertrude Bell and Voltaire

Gamurrs

• Basement Brothers - Almost monotone narration, but possibly the only channel in english that plays and shows several details of old Japan exclusive PC games (PC88 and PC98 mostly)
• Russian Badger - There's a very high chance you've seen some of the memes straight out of his videos. Start with "Heavy flamer heresy" if you've never seen any.