Celebrim
Legend
I'll suggest my candidate for The Best Thing Since Succeed/Fail: Modos 2 (on DrivethruRPG). By leaving the basic resolution roll open to interpretation, it encourages GMs and PCs to be creative even when they roll low. And the rules are laid out discretely in a catalog, which should facilitate rules-smithing.
It's a cheap/free one, though. It might not scratch your spending itch.
I'm really not into systems that require large amounts of fiat by the GM. "Basic resolution roll open to interpretation" sounds like a nightmare. Like right now some of my biggest gripes against systems are when I can't determine whether or not the difficulty of something should be "moderate" or "hard" and it's not something I thought about beforehand so I don't have a written guideline and I let the players roll hoping they'll beat the hard check or fail the moderate check and make my ruling irrelevant, only for them to beat only the moderate check and me now forced to make a judgement call. That for me is a bigger failure of the system than adding up eight modifiers to get a firm idea of difficulty. So coming from that, it's not so much that I wouldn't chip money toward free rules as much as I hate trying to run a system that is fiat with a gloss of fortune.