EntropyDB (formerly known as db4objc) is an embedded object database for Mac OS X 10.5 and iPhone OS written in Objective-C. You can embed it in your own application or you can use it for RAD (Rapid Application Development) or Rapid Prototyping for iPhone with the open source Rapid tool. 

Although not strictly an object database (it uses SQLite as the underlying persistence engine) EntropyDB mimics the db4o API and licensing scheme so we list it in the projects section.

http://code.google.com/p/entropydb/