Objective C ORM
UPDATE: DBAccess has now been replaced by the open source ORM Shark.
We have recently released a free to distribute and use ORM for Objective-c / iOS called DBAccess, which can be downloaded from http://db-access.org/, it has taken several years to develop and has now been deployed within many applications.
The documentation is fairly basic, but is slowly getting created/updated and we welcome community feedback and suggestions.
There is also a roadmap available to detail what features are going to be released in the up-coming releases.
It may not be for everyone, but I hope our effort will help some in the community.
NOTE, I have been alerted to the fact that it is bad form to promote your own products in a forum such as this, but I just wanted to add that I am only posting this in answer to the OP because I feel it does bring something useful to the table and I sincerely hope it will prove helpful to some people, even if it's not for everyone.
UPDATE: DBAccess has now been replaced by the open source ORM Shark. It is API compatible, and is available on github. http://sharkorm.com and https://github.com/sharksync/sharkorm
Thanks Adrian_H
There are a lot of alternatives out there, but in my opinion, nothing is better than Apple's Core Data. It is easy to use and implement, and is incredibly well documented (as everything that comes from Apple).
One excellent resource to learn Core Data quickly and use it right away is Paul Hegarty's CS193P videos available on iTunes U, which you can download using the following link:
http://itunes.apple.com/WebObjects/MZStore.woa/wa/viewPodcast?id=480479762