Google has open-sourced C++ B-Tree, their C++ template library for implementing B-tree containers with similar interfaces to the standard STL map/set/multimap/multiset containers.
B-trees are commonly used for databses and file-systems but the data structure can also be used well in a smaller capacity for storing data in system memory. Numbers published by Google show B-tree containers as being able to reduce memory usage by 50~80% over Red-Black tree containers. When dealing with large data-sets, B-trees can also save memory while improving performance.
The open-sourcing of this Google C++ template library was announced on the Google Open-Source Blog
. The code can be found on Google Code as cpp-btree