Ruin Explorer
Legend
Stars Without Number is good, though I feel like there's probably some design improvements from Worlds/Cities Without Number that could be backported into it.I don't think Stars without Number is d20 based but it's pretty close as an OSR game and I've heard good things.
You are correct that it's not d20 though - it uses d20 for attacks etc. like D&D, but 2d6 for skill resolution and so on. It does however actually work a lot better than most d20 systems for skill checks as a result! Soz d20s, you're not suited to all tasks!