LevelDB
A fast key-value storage library
...Written at Google, it contains keys and values that are arbitrary byte arrays, ordered within the key value store according to a user-specified comparator function.
LevelDB is not an SQL database and does not support SQL queries or indexes. It simply provides an ordered mapping from string keys to string values. Data is stored already sorted by key, though callers can override the sort order through a custom comparison function. It supports forward and backward iteration as well as transient snapshots, and automatically compresses data via the Snappy compression library.