I would say lambda, either in boost or in C++ 0x. this is not something people will find on their own most likely, but I think it allows to reduce code and maintenance significantly.
Plus changes the way you look at programming in certain way. moreover, it gives solid introduction to functional programming.
for example, you can sort collection very concisely using some weird requirement:
std:: sort(begin, end, lambda::_1 + lambda::_2 > 0);
I would also add template expressions. I am currently playing with them, they are powerful tool to produce very efficient code while maintaining very close resemblance to problem description. plus, I do not think any other language has similar facilities. http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Expression-template