Reading code is one of the best ways to learn new idioms, tricks, and techniques.
Sadly it's very common to find badly written C++ code. Some use C++ as it was C, others as if it was Java, some just shoot in their feet.
I believe gtkmm is a good example of C++ design, but a binding could not be the better code to read (you need to know the C library behind that).
Boost libraries (at least the one I read) tend to be less readable than I'd like.
Can you mention open source projects (or other projects which source is freely readable) that are good example of readable, modern, well-documented, and auto-contained, C++ code to learn from?
(I believe that one project per answer will be better, and I'd include the motivation that led you to selecting that one.)
