Berkeley DB (libdb) is a programmatic toolkit that provides embedded database support for both traditional and client/server applications. It includes b+tree, queue, extended linear hashing, fixed, and variable-length record access methods, transactions, locking, logging, shared memory caching, database recovery, and replication for highly available systems. DB supports C, C++, C#, Java, PHP, and Perl APIs. It supports key-value pair (NoSQL), SQL, and Java Object formatted data. It is available for a wide variety of Unix platforms as well as QNX, Android, Mac OS X, and several varieties of Windows.
MoCo is a system that monitors and controls SCADA points, such as temperature, HVAC, power, lighting, sprinkler systems, and more. It has industrial capabilities, but is scalable from very small home systems to enterprise, world-wide ones. It works with inexpensive I/O bridges (including X10, Insteon, and 1-wire) from a number of manufacturers. It has sophisticated history logging and diverse control capabilities, as well as a 117-page implementation manual.
wave-player is a remotely-controllable music player (frontend) connecting via HTTP calls to a music database (backend). It allows music to be controlled and streamed via HTTP so you can hear your music at work or anywhere else. The frontend is based on QT4 and the backend uses via Perl/MySQL. The API is open and REST-based.