Frameworks that work for both iOS and OS X

CocoaLumberjack by Ernesto Rivera. A logging framework. I use this.

Ensembles by Drew McCormack. A synchronisation framework for Core Data.

Extensible iOS and OS X animation library called pop. By developers at Facebook under BSD license.

Frameworks for OS X

 

Frameworks for iOS

PromiseKit by mcxl. Makes working with asynchronous design patterns easier.

Quadrilaterals on CALayer

Bridging AGGeometryKit with POP for amazing dynamics and animations.