We use continuous integration to help keep the quality of code up. For this, we use a tool called Jenkins. Jenkins runs through all our tests to make sure we haven't broken anything.
We also have a developer wiki with a whole bunch of information on how to get started with development, coding standards, and the like.
Of course, there's the source code. Applications would be nothing without the source code. Our source code is hosted on Launchpad, along with the bug tracker.