Index Card RPG: a d20-based system by Hankerin Ferinale (RuneHammer). Technically classless and levelless. It uses modifiers for stats. Point buy your attributes and a few other abilities, including your armor, weapon damage, and effort.* Your items give you further bonuses. Also it generally applies a singular difficulty rating for an encounter. So a room may have a difficulty of 13, meaning every task (disarming a trap, stealthing, perceiving, etc.) and monster AC in this room will be 13. But this can be adjusted for easy things in the room (-3) or challenging things (+3).
* Effort is kind of like damage for non-combat related tasks. A chest may basically have the equivalent of 10 HP. So the rogue who is unlocking the chest, may require multiple successful rounds to unlock the chest. With each succesful Dex check, allowing them to dwinde the chest's "HP" down to 0 using their effort rolls.
13th Age: Nice mix of 3E and 4E. It's lighter than Pathfinder but still rules medium.
Shadow of the Demon Lord: a d20 system by Robert Schwalb that is similar to 5E, but with more complex of character build options. Pick any Novice path (priest, warrior, magician, rogue). Then you can pick any Expert path (e.g., wizard, artificer, ranger, fighter, druid, cleric, paladin, warlock, etc.) no matter what you picked for your novice path. But what! There's more. And this also includes being able to pick any Master path (e.g., Necromancer, Bard, Illustionist, Mage Knight, Cavalier, Acrobat, Gunslinger, etc.) or a second Expert path. Mix and match to your heart's content across 10 levels. Every level is the completion of an adventure of the appropriate tier (1-3; 4-6; 7-9). The default setting is a bit dismally dark and cynical, but you can easily strip it away. Oh, and I think that most tasks are resolved by rolling over 10.