You may have heard of the Cheddar to do list app from Sam Soffes that gained some notoriety on release because of its simple interface, and capability of syncing between different devices. Recently the source code was also released.
This is a particularly good source code example to learn from demonstrating how to utilize Core Data, communicate with RESTful web services, custom user interface components, in app purchases and more.
You can find Cheddar over on the open source apps list.
