Zak S is a well known figure in the OSR, and one I enjoy reading most of the time. I don’t usually bother including his posts here, but he’s gone above and beyond with a 10 part art history overview that’s probably 10 times better than anything you’d get in school, if only because he ties it in with D&D, and also doesn't spend any time worrying about the minutia that no one reading his posts cares about. This post is the overview of the course. I'm linking to it here for myself as much as for you.
Eric over at the Dragon’s Flagon takes a look at hit point modifiers for old school monsters. You know, those random +1 or +3 hp that monsters got for no really good reason. Well, Eric figured out a really good reason for them! Seriously, if you're playing an old school game, you need to read this post.
Tieflings are an interesting race. Demon-touched humans… Like many, I’m not a big fan of their appearance in 4e and 5e. I like that they’re included in the game, I just don’t like the art direction.
If you want to randomly roll some demonic features, check out this random chart.
If you're looking for more visual inspiration, this is a nice collection of images from Tales of the Grotesque and Dungeonesque that better suites my mental image of what they should look like.
Gus over at Dungeon of Signs takes a look at OD&D’s orcs, and comes to the conclusion that orcs are actually the result of some sort of disease! It’s an amazingly flavorful take on an old standard, and one I'm stealing for my table!
Did you know 5e works with the ACK economy rules? Paul at Blog of Holding did, and now you do too!
So the party found a secret door, and you don’t know what’s in the room beyond? Elfmaids and Octopi has you covered.