The situation regarding Arc is somewhat... confusing. There is the official Arc 3.1 which is developed by Paul Graham and Robert Morris. It has not been updated in quite some time now.
There is the unofficial Anarki port[1], which has seen active development.
There are various ports of Arc to other languages, such an jarc[2] and Rainbow[3]. These are generally the same as Arc 3.1, but may have new features and may not be 100% backwards compatible.
There is also ar[4], created by awwx. It is essentially a rewrite of the Arc compiler from scratch, trying (more or less) to keep backwards compatibility while also adding new features. Its goal is to make the compiler accessible to Arc code, allowing for more hackability. This is what I personally use.
Then there are all the various other ports, forks, private repos, etc... not to mention spinoffs and libraries like wart[5], Lathe[6], or Arubic[7].
Basically, Arc is what you make it. Its goal is to be hackable, so that you can change the language itself. Thus the distinction between "official" and "unofficial" is somewhat less important than it is in other programming communities[8].
* [8]: The distinction may be less important, but it does still matter: if you want your code to be runnable by the widest possible audience, you need to be careful to write your code so that it works on at least Arc 3.1, and probably Anarki as well. This doesn't matter if you're targeting a single platform.
