As of today, the Activiti source code lives on GitHub: https://github.com/Activiti
We’ve been using git and GitHub for quite a while already internally, and we’re really pleased with all the benefits it has over SVN (which is now the CVS when SVN was the new hot thing ;-)). Besides being way faster and easier to merge, GitHub allows you and other contributors to send us in a much easier way patches using forks and pull requests (see https://help.github.com/articles/using-pull-requests).
For issues, we will still be using Jira on http://jira.codehaus.org/browse/ACT, because in (our humble opinion) Jira is more powerful (eg. filtering) than GitHub issues (plus, there is no easy way to ‘migrate’ all our jira history, as there was for the svn commits). The forum remains on http://forums.activiti.org/en/index.php. Our Fisheye functionality will be taken over by the github ‘network’ and ‘commits’ tabs.
Don’t forget to star the Activiti repo on GitHub and create your own fork today! Get that pull request machine rolling!