This same company offers several frameworks in their Developer Resources site based on the code they written for their own Mac OS X and iPhone applications. Here is a brief list of what the offer as of today:

OmniBase
This is the lowest level framework used in every product they build.

OmniFoundation
An extension to Apple’s Foundation framework, including string scanner and regular expression processing.

OmniDataObjects
CoreData-like API, built on top of SQLite. This framework provides the persistence for OmniFocus on the Mac and iPhone.

OmniAppKit
Set of extensions to Apple’s AppKit.framework.

OmniInspector
Inspector framework used within OmniGraffle, OmniFocus, OmniOutliner and OmniPlan.

OmniNetworking
Objective-C wrapper offering many networking constructs.

As of yet, I have not tried any of the frameworks in an iPhone application. If anyone has experience with these as it relates to the iPhone, please post a comment and share your insight.