PSTCollectionView is an open Source, 100% API compatible replacement of UICollectionView for iOS4.3+. PSTCollectionView is presently no longer in development but is still a great tool overall. This tool supports animations, and works with Xcode 4.5.2+ and ARC.
RaptureXML is a simple, block-based XML library for the iOSplatform that provides an expressive API that makes XML processing easy and fun. RaptureXML changes the game when it comes to XML processing in Objective-C.
RKValueTransformers is a powerful library that provides a simple value transformation API in Objective-C. RKValueTransformers has a core set of transformers that can be customized and new transformers can easily implemented to meet the needs of any application.