Weirdly I find the v8 source and jQuery (at least several years ago) to be the best. They're examples of real-world, multi-distributed-collaborator, mature proj

Weirdly I find the v8 source and jQuery (at least several years ago) to be the best. They're examples of real-world, multi-distributed-collaborator, mature projects. Harder, I think, than writing "clean" code is maintaining a large code base's organization, distribution, etc and these are really good examples of that. (d3 and three.js are also very interesting to read, but they're not quite in the same class as the former.)