Twisted.

Twisted.

1. No shortage of things to do. We have hundreds of open tickets, with difficulty that varies from trivial fixes to substantial new systems.
2. Extensive feedback on every contribution. You'll learn a lot about how to write better code.
3. Thousands of unit tests, run on lots of different platforms. You'll learn how to write good tests and keep your code portable.

And of course, being an open source project we're always looking for more contributors, so we will be happy to have any help you would be inclined to give.