Or Tunnels & Trolls, which started out as a simplified alternative?
I don't think either fits the classic definition of a Fantasy Heartbreaker, though, because I always understood that a FH has to be a "let's do D&D, but better!" game which not only fails to innovate substantially over D&D, but represents both a notable labor of love/major investment on the part of the creator, and a commercial flop.
RuneQuest did spring at least in part from the Perrin Conventions house rules for OD&D (first debuted at DunDraCon I in '76, I believe), but it's done reasonably well as a game system (and, I think, does a lot of stuff differently from D&D).