PGCocoa a thin Cocoa wrapper around the PostgreSQL client library. It provides Objective-C objects for interacting with the database, including boxing of intrinsic data types. It is complementary to Core Data; it doesn't provide anything like Core Data's infrastructure, but PGCocoa would be useful for implementing an NSIncrementalStore that interacts with a PostgreSQL database.
This is a work-in-progrss, but it is far enough along to useful, so I am making it available under a BSD license.