Currently, I am participating in an open source project, VCD (Video Content Description), leaded by Vicomtech. Its a metadata language (Python API, JSON schema) for complex labels, including ability to label objects, actions, relations, etc. Its devised to support labels for multi-sensor set-ups (e.g. a set of cameras and LIDAR from an instrumented vehicle).
You can find more info at the VCD webpage.
In my Github page you can find some other repositories I’ve created or participated in:
There are some legacy repositories in sourceforge, although these are at least 8 to 10 years old (from 2010-2012), so use with caution!