One of the best ways to learn about programming is reading high quality code/projects from great engineers. Which open-source projects do you think is worth looking at? I mean, that code that you can print and sit under a tree with a glass of wine and enjoy reading.

If you can, also specify if the software is great to look at because its documentation, design, UML diagrams or just plain code. I believe UML is not very common within open-source projects.

Is there such a thing as a project branch that polishes code and design with the sole objective to give other programmers a great example of great software?