DrivethruRPG and RPGNow run a sale right now until July 31. Christmas in July. Here are some recommendations in alphabetical order.
Bookkeeping Notes: I get commissions for purchases made through links in this post.
Abstract Dungeon: a fast-playing and versatile narrative fantasy game. It is beginner-friendly and comes with a resource-spending mechanism. Review here.
Barbarians of Lemuria: Mythic Edition: Sword & Sorcery at its finest - wonderful career system for character creation and a pulpy lightweight ruleset by Simon Washbourne.
Barrel Rider Games' White Box: this is for you if you are a fan of the original RPG from 1974. Here is a review of the White Box Omnibus.
Beyond the Wall and Other Adventures: beginner-friendly teen OSR game in the vein of Earthsea - with playbooks. Full review here.
Crimson Blades 2: Dark Fantasy RPG: modified Swords & Wizardry ruleset (only uses d6) with a grim fantasy setting by Simon Washbourne.
Crypts and Things Remastered: Sword & Wizardry ruleset geared towards Sword & Sorcery. Comes with its own setting.
Death is the New Pink: a post-apocalyptic Tank Girl game by Mike Evans with a pared down OSR ruleset (Into the Odd). I haven't bought it yet but I have high respect for the author.
Fever-Dreaming Marlinko and Misty Isles of the Eld - two OSR products - are probably enjoyable, too. I liked Slumbering Ursine Dunes and have only heard good things about Hydra Cooperative products.
Hollowpoint: play an agent when the shit hits the fan - a narrative, ingenious game. One of my all-time favorites. I have a review, but it's in German.
Hubris: A World of Visceral Adventure: a toolbox and weird fantasy setting for old school games - wacky, bizarre. Full review is here.
Mythic Game Master Emulator: an oldie, but goldie when it comes to solo role-playing. A tad too cumbersome for my taste but still useful. The gold standard if you want to say so.
Scarlet Heroes: the game for solitaire OSR gaming, choke-full of great ideas which you can use for other D&D games, too. A must-get.
SUPERS Revised Edition: a lightweight and robust superheroes RPG. More about this here.
Symbaroum: beautiful dark fantasy setting, wonderful artwork, and a robust (but not narrative) ruleset. More about the game here.
The GameMaster's Apprentice: Base Deck: cards for solo role-playing and game mastering aid - create NPCs, plots, etc. Review here.
Wormskin: dark fairy tale setting published in zine format for B/X. More about this here and here.