One of my most used is not in Boost proper, but the Adobe Source Libraries (ASL) built on top of Boost — specifically, the extensions to the standard algorithms that accept a boost::range in place of separate begin/end iterators. Then instead of calling, say,
std::for_each(some_container.begin(), some_container.end(), do_something());
I can simply say
adobe::for_each(some_container, do_something());
(I do hope these parts of ASL migrate to Boost eventually.)