LevelDB: a fast and lightweight key/value database library
Note that this is not a database <i>server</i>, like Redis or Memcached. This is a database <i>library</i>, more along the lines of sqlite. In particular: "There is no client-server support builtin to the library. An application that needs such support will have to wrap their own server around the library" and only one process can access a database a time.