MDBM is a super-fast memory-mapped key/value store.
To use the modern buzzwords, it is NoSQL, and for many operations, it is Zero-Copy.
It is based on an earlier version by Larry McVoy, then at SGI. That in turn, is based on SDBM by Ozan Yigit. wikipedia DBM article
Yahoo added significant performance enhancements, many tools, tests, and comprehensive documentation. It has been used in production for over a decade, for a wide variety of applications, both large and small.
It is being released under the BSD license.