We are in the process of moving the code and associated documentation to the Apache infrastructure. This page still defines the canonical repository and will be updated when that changes.
The wave protocol code, including the models, operational transform and example client and server are all hosted at the Wave Protocol code project. To date, we have open sourced several components related to Google Wave, which amounts to more than 200K lines of code.
The default repository hosts the prototype client and server projects. It also contains re-usable components for any wave system, including Wave's operational transform (OT) implementation, the wave model, concurrency control stack, and rich text editor. See the OT whitepaper, model code walkthrough for more details. We are building these towards the Wave in a Box milestone at the moment: http://code.google.com/p/wave-protocol/source/browse/
There's an alternative, lightweight Wave client, called Splash, which is built on the Wave Data APIs: http://code.google.com/p/google-wave-splash/
First, get Java.
Then browse to http://localhost:9898.
We welcome patches to the code. If you're looking for something to work on, check the starter projects wiki.
When your patch is ready, read the code submission instructions. Check the continuous build before submitting.
The following pages provide more information: