WordPress is an excellent example of a well-run Free Software project.
Public pages on coding standards, WordPress IRC, and reporting bugs are clear, comprehensive, and elegant. Installation and upgrading are straightforward: explained in both “5-minute” and “detailed” forms. They use Trac, an awesome bug tracking system.
I can’t say much about the source except to say that what I’ve seen is simple and coherent. Database interaction, for instance, is abstracted in a way that makes sense. Ohloh has some more useful details and statistics.