Ask HN: What are the best open source projects to learn from?

I see a lot of "how not to do X" and "you should never do Y", as well as sites dedicated for bad practices and anti patterns.

- ShitHub http://news.ycombinator.com/item?id=4622805

- CodeCrap http://news.ycombinator.com/item?id=4622630

So I know now a lot of ways how not to code, but I much rather learn how to code, than how not to.

My question is, what are some widely agreed projects to take example from? (either written in Java, Ruby, Python, JavaScript, Scala) based on the following aspects

- good use of the language - great error handling and logging - architecture - testability - concurrency - performance - MVC patterns - security - API design - naming conventions

What are the masterpieces of code of our modern day, that every programmer should know about? Or if we stick to the what not to, which famous projects I should not look into as an example?