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

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

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