Apache Wave, where Wave development happens at Apache. Wave is a rich, distributed, near-real-time collaboration platform, which allows users to work together in new and exciting ways. The main sub project of Apache Wave is "Wave in a Box", a stand alone wave server and rich web client that can serve as a Wave reference implementation. As the project grows we hope to expand to offer other Wave related goodies as well.
Walkaround is a variant of Wave, based on the Apache Wave code base, that runs on App Engine. Walkaround can import waves from wave.google.com to allow users to keep working with their data regardless of the future of wave.google.com. (The import feature is still experimental.)
If you're just learning about Google Wave, you may wish to also check out http://wave.google.com/. You may also want to read about the federation effort in the introductory blog post. You can browse the site to learn more about the community principles, review the draft protocol specifications, read the architecture white papers and the design proposals.