The majority of plugins that we use within / alongside our sites comes are stored on Github. This has become a huge central hub of resources for all things dev and there are a number of reasons why. One of the biggest contributors to its success is its ability to version control with the public. I have found a number of times that when I am having issues which a certain plugin which I have sourced from GitHub, there is no doubt that someone else has had similar issues. More often than not, simply visiting the resources issues section on their GitHub page is enough to help you out.
With GitHub being so community driven and very open to public access, many people post their issues but also post branches of the plugin with fixes they have come up with themselves. This is fantastic as it not only helps other people using the plugin out, but also gives the developers of the plugins vital feedback and bug fixes which they can then push to the core version.
At Shape we use GitHub to store our frameworks privately also so we can each add little features as and when we like so that the rest of us can benefit from it at a later date. Jason is pretty handy at creating plugins for Craft these days too and has been publishing them on ourGitHub so go take a look!