sipPQ is an interface to PostgreSQL's libpq. It is written using SIP, and is being used in a production environment. The original author is Jonathan Gardner.
sipPQ only tries to interface directly with libpq. A more useable interface can be written in python that runs on top of it.
sipPQ demonstrates that SIP can also handle C libraries, and quite well at that. Beyond learning how to write a sip script, not much effort was required to create a finished product.