Open Source Software is a computer Software whose source code is available publically to use, modify and re-distribute under different licenses.
Learn from OpenHatch Training Missions
Let's try to search from Open Hatch projects
They all use a bug tracker
Most of the projects have tags like Mozilla uses good-first-bug, OpenHatch uses Bite-size
They have some awesome people willing to help you!
No matter what programming language you know, there is a project for you in Mozilla ;)
Let's try to search from What can I do for Mozilla?
Josh Matthews made a better tool Try out Bugs Ahoy
Let' try Bugzilla!
One of Google's 20% projects
AngularJS + Python
C + Python + Awesomeness ;)