I know how to manage milestones for several projects, but is there also an option (or a plugin) where I can see all (open) milestones from all projects?
edit: I use Gitlab
Gitlab 7.1 includes a new feature: Group Milestones.
Users are now able to see all milestones for different projects in the same group. It is not exactly what you wanted to do, but is a viable alternative, I think.
Reference: https://about.gitlab.com/2014/07/22/gitlab-7-dot-1-released/