I’m a big fan of free/libre open source software and the benefits and collaboration of crowd coding. As well as releasing a lot of my own projects as open source I’ve also contributed to the following third party applications:
Flubaroo
I have added significant features (numeric range answers and autograding) and refactoring to Flubaroo, an open source quiz grading script presently the second most downloaded Add-on in the Google Web Store!
BetterLog
An open source Google Apps Script logging library.