SQLite
SQLite is a relatively small C library, which provides services of a database engine. Its creator is D. Richard Hipp and the software is in public domain.
Unlike the usual client-server paradigm, the engine is not a standalone process, with which the program comunicates, but becomes an integral part of program. Therefore the primary communication protocol are direct API calls within programming language. This can have a positive impact on the amount of overhead, latency times and overall simplicity.
The library implements most of SQL 92 standard, including transactions, triggers, complex queries.