[__jal]

PG is, IMHO, probably one of the best code bases for this.

I'm far and away not a database hacker, but have read parts of the code at various times to better understand what I was seeing. The codebase is incredibly well written and organized, and the documentation (both developer and user) is top-notch.

The only problem I can think of with using it in a course would be choosing what to focus on with only a semester. Add in the long revision history and I think there are also multiple theses in there on the sociology of open source.