[jlangr]

There's a big difference between writing tests for existing code and TDD. I think it's a great idea to first try and write tests for someone else's existing code so you discover what a pain it is.

https://searchcode.com can be useful in helping find repos based on code criteria.

Were it me, I'd build something small and interesting to me, write some unit tests against it after-the-fact. Then I'd do it again, using TDD the 2nd time to see how the approaches differed.