I don't think I've ever seen any scientific computational software that I would consider "good" in terms of software design. I don't mean in terms of functionality, but in terms of good high-level design, coding standards, readability, etc. Most things I've encountered have some degree of bad code smells.
What are some examples of really good computational science software engineering? Why do you like it? Preferably open source so we can see it, any language.
To minimize the subjective nature of the question, and because there's no one "right way" to do things, please give concrete examples of things like software design patterns, agile development practices, regression testing, and similar things implemented in scientific codes.
I like this post titled "The Exceptional Beauty of Doom3's Source Code." I'd love to see a similar example in the scientific world.