Black Hack is an ultra-lite OSR RPG system that resembles the earliest versions of D&D as reinterpreted or recreated from hazy memory. It's ultra-lite in the sense that it's so simple that a person could actually hope to remember all of the rules without years of play. The core rules are on a twenty page document only fifteen of which are actual rules. It has only the four core classes: Warrior, Conjurer, Thief, and Cleric. It features the usual attributes generated by rolling 3d6, hit points, and hit dice.
It differs from most d20 games in that it uses those stats way more often. Armor class is replaced by strength in the case of melee and dexterity in the case of ranged attacks. All rolls are aimed at going under a given stat on a d20. A roll of one or two is always good and nineteen and twenty are always bad. Armor acts as extra hit points, fifth edition style advantage and disadvantage are used in a number of places, and stats have a chance of going up every level. Aside from general simplicity the philosophy seems to be that rolls are almost never modified. If you roll on a d10 you use the number you get, no plus or minus some modifier. While d20 purist are likely to find this unsatisfactory it's a lot easier to get into and makes for faster game play. It also has a number of small expansions such as Race Hack, Class Hack, Pulp Hack, Item Hack, and many more. All of these make for a fairly easy, low cost entry but lots of room to expand. If you want to see the basic rules they are available for free here or you can support the publishers by buying it here.
IRON NODER: WE'LL RUST WHEN WE'RE DEAD