state-of-the-art C++ projects
<p>I like to go through existing software projects as a source of learning and new ideas.
doing so I discover things that I did not think were possible</p>

<p>in your opinion, what is the top state of the art C++ project that you have used/develop/extended?  can you state reasons why you consider it state of the art and what you can learn from it.</p>

<p>my latest craze is boost::phoenix, <a href="http://www.boost.org/doc/libs/1_43_0/libs/spirit/phoenix/doc/html/index.html" rel="nofollow">http://www.boost.org/doc/libs/1_43_0/libs/spirit/phoenix/doc/html/index.html</a>, which is very comprehensive functional programming library.
Despite its capabilities it is straightforward and easy to extend.  After some tweaking I was able to write multithreaded lambda parallel loops and mathematical domain specific language, probably within 2 weeks.</p>

<p>What is yours?  (please do not just say boost, as it is huge collection of project)</p>
