What source code is worth studying?
lpolovets: This is half answer/half tangent, but I like http://www.google.com/codesearch a lot for browsing and reading code. A fun thing to do is to look at how others implement something you want to implement, or use some library you want to use. You end up finding a lot of different ways of doing the same thing, and if you find something that you want to explore more, it's easy to browse around.