Q:

Most Instructive Open Source Android Projects?

I'm looking to compile a list of instructive open-source Android projects.
My criteria for instructiveness:

High code quality
Good API coverage, demonstrating how various parts of the API interact
Continually improved, showing that the designs are in fact maintainable
Good aesthetic quality

In other words, the apps that as intermediate to advanced Android developers we should be looking to for guidance, and not simply tutorials that demo a few concepts.

A:

I found that I learned a lot going through the K9 Email Client.  

