Google has graciously gifted to the community a set of STL-like containers that use B-trees under the covers. The code has been released under the Apache 2 license.

C++ Containers That Save Memory And Time

We’re pleased to announce C++ B-Tree, a C++ template library that implements B-tree containers with an analogous interface to the standard STL map, set, multimap, and multiset containers. B-trees are well-known data structures for organizing secondary storage, because they are optimized for reading and writing large blocks of data. But the same property that makes B-trees appropriate for use with databases and file systems also makes them appropriate for use in main-memory, just with smaller blocks. [...]