GlPortal is a free open source 3d game. Check out a video here: https://vimeo.com/163973907
I have added a bunch of tasks to the simple tasks in our bug tracker https://bugs.glportal.de/index.php?project=1&type%5B%5D=5
If you need more information on a ticket you can message me or join our chat. http://webchat.freenode.net/?channels=%23glportal&uio=d4