SQLite (Score 1)

SQLite has very good architecture, code (fully and relevantly commented), documentation and tests. Its bug tracking is also very good. Sure, it is not a very large code base with lots of developers but it is keeping clean in its evolution. Just compare it with Firebird (which has admittedly more features): Firebird code is such a mess, from top (architecture) to bottom (name of variables). I'm not advocating one software above the other, but just saying that comparing both is interesting and shows that it is possible to make good software robust and clean instead of poor and messy.