We need unit tests for just about everything in this project. I know this isn't quite as exciting of a task... and that is why I listed it last, but it would be a major help.

It is imperative that we thoroughly test modules so that we can identify fine-grained errors. As @kuba pointed out, it will also help you become familiar with the codebase and how it works. It can enable you to contribute meaningfully without having to read the code in its entirety.

(I am currently working on writing an integration testing suite that I hope can test 100s of example servers quickly, but this only detect end problems and won't provide information about the minor errors that may occur during the course of execution)