Most beautiful open source software written in c++
c++
Solution
You could look at the source code of MySQL GUI Tools. Its written using gtkmm, and the code does some interesting difficult-to-implement GUI things.
Problem
I was told that to be a good developer, you should read a lot of other peoples source code. I think that sounds reasonable. So I ask you, what is the most beautifully written piece of open source software that is written in c++ out there? (Apply any definition of beautiful you like.)