Forking these days is mainly friendly because of the ecosystem that Git has fostered, but until relatively recently, forking was generally hostile, to force development to move in a certain direction away from the original intent.
In this case, Arduino SRL (formerly Smart Projects, the arduino manufacturing spinoff) has forked the IDE, and significantly up-revved the version number to try and get people to move away from the original team's IDE.
Technically, that was one of the more hostile forks; EGCS developers were very unhappy that gcc wasn't moving fast enough and in the direction they wanted. They put their code where their mouths were. The only significant, more-hostile fork that I know of is Emacs/XEmacs.
I can't think of any project names at the moment, but I believe that most of the pre-Github "hostile" "forks" were due to a package being abandoned.
That's what I'd heard at first, but I remember being corrected and told that it was much friendlier than the public assumed, and it was basically treated as gcc's unstable branch.
Also, remember that egcs started out as a merger of several devs' individual experimental forks containing stuff like new frontends, new architectures, and optimizations for specific processors. The projects that went into egcs were all pre-existing projects; egcs was just a consolidation.
All the competing Unix forks? The infighting in the Unix world and the blatant attempts to break compatibility with each other pretty much killed Unix, about the only thing keeping it limping along any more is all the effort that's been put into OpenBSD and FreeBSD, and the commercial backing for Solaris.
Edit: Just realized you said non-hostile. Yeah, not so much, prior to GitHub most forks that weren't to try to take ownership were either experimental in nature, to continue development on an abandoned project, or in order to develop some features that the original authors disagreed with (semi-hostile). I'm not sure you can classify picking up a dead project as a hostile action, but it isn't exactly benign either. As for experimental branches those were usually non-hostile, but at the same time if they bore fruit they would typically get merged back into the upstream so not really a fork in the truest sense of the word, but more like a really long running branch.
In this case, Arduino SRL (formerly Smart Projects, the arduino manufacturing spinoff) has forked the IDE, and significantly up-revved the version number to try and get people to move away from the original team's IDE.
See also: http://en.wikipedia.org/wiki/Fork_(software_development)