It depends what you're interested in learning about. I'm a ruby/rails developer by trade so rails is a really good place to start for me. The open source librar

It depends what you're interested in learning about. I'm a ruby/rails developer by trade so rails is a really good place to start for me. The open source libraries around it like devise, cancan, haml+sass etc are required reading if you're going to be using them regularly. I'm currently interested in C and lower level systems stuff and I've found redis to be a really readable codebase. It's not an introduction to C but provides a good example of how to manage a larger C project.