What are some of the best-written open-source projects you have seen?

Question

What are some of the open source projects out there that you would hold up as shining examples of projects that correctly and effectively use enterprise sofware patterns and best practices such as Inversion of Control, Model-View-Controller, Unit Testing, etc.?

For purposes of this question the project should:

1. Include source code that illustrates the pattern in use, and
2. Be doing something important and useful, i.e. not using the pattern frivolously just because it is flavor of the week. Hence the words, "Correctly and Effectively" in the question

It should be software that you could show to the people who work for you and enthusiastically be able to say, "I want you to do it the way these guys did it."

Answer 1

You mean Spring? Or projects that use it?

UPDATE: WebWork, Guice, and Spring Security all fit the mold.

Answer 2

Prism is very good for MVVM in WPF and Silverlight

patterns they use Patterns in the Composite Application Library

Answer 3

An objective pick would be the Spring Batch project.

How did I pick it? Judging from the technical debt as seen on the Nemo Sonar instance, Spring Batch has the lowest debt/line ratio for projects larger than 10k lines of code.

Answer 4

Most of the GNU project is very very well written, over a very long period of time, with strict guidelines.

Answer 5

When I first looked at the source code for DotNetBlogEngine, I was impressed at how well it was organized. And it didnt couple any of its components to the interface, making it extremely flexible.

It's not a terribly huge project either, not like trying to recompile your own linux kernal or something. So you can dive in quickly and have some fun with it.

Answer 6

Castle project

Answer 7

You can try this book - Beautiful Code

The author has collected some experience-sharing articles of open source projects. e.g. Python's Dictionary Implementation, Subversion's Delta Editor, etc.