Tkrzw
Tkrzw is a library of routines for managing key–value databases. Tokyo Cabinet was sponsored by the Japanese social networking site Mixi, and was a multithreaded embedded database manager and was announced by its authors as "a modern implementation of DBM". Kyoto Cabinet is the designated successor of Tokyo Cabinet, while Tkrzw is a recommended successor of Kyoto Cabinet.
Tokyo Cabinet features on-disk B+ trees and hash tables for key-value storage, with "some" support for transactions.