If you're willing to learn about patterns, jQuery and Underscore.js are excellent places to look at. If you're looking for something harder and unconventional,

If you're willing to learn about patterns, jQuery and Underscore.js are excellent places to look at. If you're looking for something harder and unconventional, see React source. It also has some low hanging fruit for pull requests. If documentation and comments are important for you to learn, Backbone.js is way ahead of everyone else in those terms. I would also look at smaller indie projects. The source is smaller, forking and adding something is easier. Pull requests are easier too.