DriveThruRPG is having a HUGE sale on thousands of titles with the May D&D Sale, with everything Dungeons & Dragons, from Original to Fifth Edition!
The sheer amount of titles available makes it hard to create a list of OSR recommendations, so this time I'll focus on Old-School Essentials as it is my current go-to ruleset.
Of course the list begins with the core OSE books, both the Classic Rules Tome and the Advanced rules. If you're reading this, you're probably already a fan (like me), but don't forget sales are a great occasion to spread the word. Tell your friends!
Well, the whole Old-School Essentials line is part of the sale, which means you can get some great adventures too, like Incandescent Grottoes or the Ennie award winning Halls of the Blood King, and also the Dolmenwood setting line!
The Black Wyrm of Brandonsford - An exquisite, sandboxy point-crawl adventure for characters level 1-3, very easy to run, with lots of interaction between locations, that I can't wait to run. Just reviewed it!
Puzzle Dungeon: The Seers Sanctum - Another very good 10 room dungeon with, well, a lot of well conceived puzzles. I've reviewed it here, and if this dungeon is a great opportunity to connect your existing campaign to the setting outlined in the best seller Planar Compass zine series which is on sale too!
Xanadu - Another acclaimed third-party Old-School Essentials dungeon adventure, by Singing Flame.
Bottomless Pit of Zorth - Another acclaimed adventure, with a slime theme and insane art, for characters level 3-5, which is a less frequent level bracket in comparison to level 1-4, and makes it definitely worth checking.
Tannic - A just released level 1-3 adventure set in a haunted forest, by Amanda Pratt, that looks pretty cool (seriously, check the preview).
Sourcebooks, Supplements & Zines
The whole Third Kingdom line of hexcrawl supplements is out there, including the great Filling in the Blanks guide.
The Old School & Cool zine series by Knight Owl Publishing.
For those who feel like reading thousands of pages, the Castle Oldskull Megabundle is definitely worth a look (includes guides, classes, bestiaries and even novels), and price-wise it is the sale with the largest discount.