Several years ago PCG reported on the early stages of a project by programmer James Lambert, who really loves the N64 and decided for the hell of it to make a version of Portal that runs on hardware first released in 1996.
Well, Lambert is clearly the type of chap who commits to something and then follows through, because he’s spent the time since then continually adding bits and bobs and now has something which, to my admittedly uneducated eyes, is the most impressive homebrew game I’ve ever seen.
Valve has been known to make fangames based on their IP available through Steam. Black Mesa is a good example.
Of course, this would have to be distributed with an emulator, but N64 emulators don’t require any proprietary code to run.