(Making it reblog-able so that it’s easier for people to add on.)
All of the blogs on the Tumblr Spotlight are highly recommended. Out of them, I follow Eff Yeah Asian History, Medium Aevum (the Middle Ages), Historicity (was already taken),Historical Nonfiction (for more general history and light reading), and a couple of others.
Also, if you look under the History tag, there’s a list of Editors and Contributors who are also worth looking at.
European history: History of Europe, The Stuart Kings, Histoire (French history), Medieval,Victorian Era Fangirl Guide, Irish History, there’s a lot of them. I’ll add some more later.
General (they tend to be Euro-centric though): It’s Johnsen (pictures!), collective-history,Lost Splendor, F-YEAH History!, I would argue, History: the Ultimate Fandom.
World Wars: The World at War, Once Upon A Time in War, Greatest Generation, WE HAVE TO GO BACK. There are way too many World War blogs to list.
Third Reich-centric: Fuck Yeah! Albert Speer, Onkel Speer (apparently a lot of people like Albert Speer), We will be victorious, they will not control us, wafflekrieg, Panzerkampfwagen VI, Ein Volk, ein Reich, ein Fuhrer, Angriff!, and more…
Miscellaneous: Art of Swords (I love this blog so, so much), Women at War, Cool Chicks from History, FUCK YEAH HISTORY CRUSHES, Fuck Yeah, History Foes! (funnier than its crush-counterpart), Latin American History, Fuck Yeah! and naturally, Fuck Yeah, History Major Heraldic Beast.
Ancient History/Archaeology: All Mesopotamia, OMG that Artifact!, Babylon Chronicle,Archaeological News, Historiated (not ancient, but a lot of pretty things).
Art: Centuries Past, Ugly Renaissance Babies (hehe…), deadpaint, wtfarthistory.
America: Founding Fatherfest, Revwarheart (a lot of other history/culture, too), Encyclopedia Virginia (I need suggestions on this section).
You can also check out archives, libraries, and museums like U.S. National Archives, LBJ Time Machine, New York Public Library, and Today’s Document.
I feel like I’ve left out a lot of great blogs. If anyone has a blog to recommend, don’t hesitate!
Recommended by others:
Sovietico, PPSh-41 - recommended by Fuck Yeah, History Foes!; both Russia-centric.
The Devils Guard, Reparations - by Panzerkampfwagen VI; Third Reich-centric.