Regardless of the location, I think using a Git repository really is the best option. It will allow people to practice with a widely used and sane tool, without being afraid they'll break anything.
I also like Github because you can create a team there and put projects under that team, and you can easily configure what team members can push to what branches. That way, we can allow members to freely push to any feature branches they're working on, and require pull requests to stable branches.
Can't we start simple? It is not so easy to come up with a project, that is innovative and accessible to many. But I guess everyone has written a handy class, no matter what about, that could be sent in into some github collection. Points at hand: good javadoc, clear description, clear code. We can all try to learn from it, come up with some improvements, it doesn't matter what the subject is, and maybe we can distract some ideas for a follow up project. Hopefully we will see a broad spectrum of ideas and codes and styles.
There are three kinds of actuaries: those who can count, and those who can't.