Berkeley DB (BDB) is undoubtedly the workhorse among the opensource embedded database engines. It started as a university project in the mid-eighties and was further developed by Sleepycat Software, until it got acquired by Oracle in February 2006.
I had the impression that BDB had lost a lot of its popularity among opensource developers to SQLite in recent times, which has evolved into becoming the default choice for developers looking for an embedded data store. I'd assume primarily because the code is not released under any particular license, but put in the public domain (which makes it very attractive for embedding it into one's code), and also because it's lightweight, supports SQL and has interfaces to a number of languages.
Of course, SQLite has its limitations and use cases (as every product), so it may not be suited for some particular application. As the SQLite developers put it: "SQLite is not designed to replace Oracle. It is designed to replace fopen().".
Yesterday, Oracle announced a new version of BDB. One of the notable features of this release is the introduction of a new SQL API, based on SQLite. According to Gregory Burd, Product Manager for Berkeley DB at Oracle, they did so by including a version of SQLite which uses Berkeley DB for storage (replacing btree.c). I think this is a very smart move – instead of introducing a new API, developers can now easily switch to a different storage backend in case they are experiencing issues with the default SQLite implementation. So now MySQL isn't the only database with different storage backends anymore 
I am curious to learn more about how the BDB implementation compares against the original (both feature- and performance-wise).
Oh, and this is actually not the first time someone put an SQL interface in front of Berkeley DB – BDB was the first transaction-safe storage engine that provided page-level locking for MySQL in version 3.23.15 (released in May 2000). The InnoDB storage engine was added some time afterwards (MySQL 3.23.34a, released in March 2001).