It must be an urban classic by now, but I can’t stop marvelling at free league’s forbidden lands. The system is dead-simple, yet leaves all the room for nuance and tactics. The fantasy world is classic, but with a surprise twist here. The official campaigns are superb with lots of “aha!” Secrets to discover, while at the same time a low-prep gameplay with just travel (well done hexcrawl) and randomly generated sites is also possible.

Recommending some free game systems of some different flavors - because free is best for people who are interested in exploring OSR.

Worlds without Number (and other games from Kevin Crawford). Great free version with basically the whole rule system and some great generators. Has a bit more character creation crunch than other systems so it can be a good transition from 5e

Basic Fantasy Rpg. Retro clone with some ‘modern niceties’ like ascending armour class. All the rules, supplements and some great adventures are available free on their website.

Mausritter. Rules light (into the odd based) system with a pay what you want version. DnD broken down to it’s simplest core where you play as cute mice adventurers.

If you want to recapture the feel of early D&D, ShadowDark (quickstart book) looks like it’s going to be pretty good. The simple tweaks to the current game make a big difference, and the artwork is spot-on. This is the image grabbed my attention when I fist saw it (and realized it was new).

Dave Trampier is the most obvious influence, but the art also reminds me of Garry Chalk’s illustrations for Joe Dever’s Lone Wolf books and the early editions of Talisman. Now that I mention them, Lone Wolf game books and the original Talisman board games are worth familiarizing yourself with if you really want to get into the old-school experience…

Going away from Fantasy we have

Hulks & Horrors a BX like space opera game

S&W White Star an OD&D Space Opera Game with a great skill system

Mecha Hack - Black Hack with Giant Mecha Robots

Seattle Hack - Black Hack in a Shadowrun Like Enviorment

Esoteric Enterprises - OD&D, Shadowrun, and White Wolf Vampire had a baby

Agents of the O.D.D. Into the O.D.D. and XFiles had a baby with a dash of James Bond

White Lies - OSR James Bond meets XFiles

NSR/Rules Light

Mausritter: I’ve run this one in various one shots for probably around 50 different players already, and so far every single one has enjoyed it. The inventory system is great, the setting is great, the hackability is great. Like most rules lights I don’t like running campaigns in it, but for pickup games in my FLGS it’s my nr 1 favourite.

Black Sword Hack: If you want to get the feel and vibe of old school pulp action sword & sorcery fantasy in a rules light way, this is THE system. After Mausritter this is my other go-to for pickup games and other one-shots.

Pirate Borg: Pirates are awesome. The Mork Borg ruleset is decent. This game combines them into a really sweet blend. Also, for my tastes, Pirate Borg adds just enough meat to the bare-bones rules-light games that this is actually the only NSR game I enjoy running actual campaigns rather than just one shots in.

OSR

OSE Advanced Fantasy: As far as OSR goes this is the holy grail for me personally. B/X is right where the sweet spot between complexity and rules-light fits in my personal experience, and OSE is the best possible reference material to use in the modern day for B/X. B/X also has 40 years of homebrewing history, and so anything you don’t like about the system should be trivial to change.

Hyperborea: Where Black Sword Hack is THE system for rules-light old school pulp action sword & sorcery fantasy, this is THE system for slightly deeper old school pulp action sword & sorcery fantasy. For me personally it has some unneeded complexity as it leans more to AD&D than B/X, but I still enjoy it for all the flavour it has.

ACKS: Way too simulationist for me when you go all the way to the nitty gritty, and there’s some weird choices made in how saving throws and attack rolls are handled, but the classes are just chef’s kiss and they are 100% compatible with B/X or OSE. If you want to run a race-as-class kind of game you owe it to yourself and your players to have a look at how ACKS handles these classes.

Stars/Worlds/Cities Without Number: Beside B/X this is my most played OSR system. I don’t put them higher on this list because I personally don’t like skill lists and feats, which are both very important to SWN/WWN/CWN, but my players love these systems, and if you’re one of those GMs who has players that come from 3rd/4th/5th edition D&D and you wanna convince them to play OSR games, this is in my experience the best way to go. It has all the customisation options that modern players (and min-maxers) love, but still with sufficient OSR sensibilities. Also, even if you don’t run this game as is, I recommend checking out the Game Master chapters on how to run a sandbox campaign, how to manage factions, etcetera. That material is absolute gold.

Other Old-School Inspired

Forbidden Lands: It’s got some flaws like every other system, but I like the dice pools, I like the way defenses work in this game, it’s got some amazing random tables and survival mechanisms. I’ve recently finished a 2 year long campaign in this system and I don’t regret it at all. If you wanna lean really heavily into the hexcrawling part of Old Shool Gaming this one definitely is a great choice.