Find a battle-tested library for the problem you're solving right now.
In C++, writing a loop that iterates over a sequence is tedious. We can either use iterators, which requires a considerable amount of boiler-plate, or we can use the std::for_each() algorithm and move our loop body into a predicate, which requires no less boiler-plate and forces us to move our logic far from where it will be used. In contrast, some other languages, like Perl, provide a dedicated "foreach" construct that automates this process. BOOST_FOREACH is just such a construct for C++. It iterates over sequences for us, freeing us from having to deal directly with iterators or write predicates.
The Boost.Geometry library provides geometric algorithms, primitives and spatial index.
The BGL graph interface and graph components are generic, in the same sense as the Standard Template Library (STL).
The PBGL graph interface and graph components are generic, in the same sense as the the Standard Template Library (STL).
Standard library extensions for simultaneous min/max and min/max element computations.
Voronoi diagram construction and booleans/clipping, resizing/offsetting and more for planar polygons with integral coordinates.
A new infrastructure for generic algorithms that builds on top of the new iterator concepts.