Anarki, as the github fork of arc is called, is a very popular version, as it comes with several libraries and tools which can be very helpful. Basically, when anyone creates an addition to arc and can't wait for pg to adopt it, they put it in Anarki. Currently it's a little bit out of date, because it hasn't caught up to the new arc3, but it's getting there.

The closest thing to the CPAN equivalent would be github, using something like CatDancer's lib function. Unfortunately, while that makes it easy to share libraries, it doesn't make it so easy to find them. That's why many of the libraries are just pushed into the lib folder in Anarki.