I sometimes hear Worlds Without Number being mentioned, but it's usually as a world-building tool, since the main book has lots of good tables for that. I rarely hear anyone talking about it as a game.

I only really learned what it is recently. It's a lot like Old School Essentials but with a proper skill system (you can choose which skills to increase when you level up, etc) and a feats system (although they call them "foci") which you can use to customize your character.

It does have a weird class system, though. It's based on three base classes: Mage, Expert, and Warrior. There's also Adventurer, but what that basically means is being multiclassed. So from those three classes you customize your character to become more specialized. Want to be a monk? Sure, pick the Warrior class (or the Expert class if you want more skill points) and then pick the Unarmed Combatant feat. It gives you better unarmored AC, and makes your unarmed attacks basically as good as a sword, etc. There are other foci which basically turns your Mage into a cleric, and so on.

I also find the combat to be better than, say D&D 5+E, with more types of actions to choose from in combat.

So, like the title says - to me, this game is great. And it surprises me that I see so few people talking about it. The basic edition, which has about 90% of the content of the full edition I think, is free on DriveThruRPG (non-affiliate link).