Golfrun is a stack oriented esoteric programming language derived by GolfScript. Indeed, Golfrun was born as a C interpreter for GolfScript (removing the possibility to escape GolfScript itself and use Ruby), but soon it started to mutate.
Golfrun is mostly compatible with GolfScript and current C interpreter has a "try to be more GolfScript compatible" switch.
Golfrun is still suitable for code-golfing, though likely GolfScript allows for more dirty tricks and thus shorter code. Since Golfrun is under development and (personal) formal definition process, it is doomed to change a bit, even if the core features and concepts will stay the same.