Cocoa-Invasion is a project mainly written in OBJECTIVE-C and SHELL, it's free.
An example of injecting custom code into a particular Cocoa application.
An example of injecting custom code into a particular Cocoa application. This injection infects Safari.app. Then you can use Objective-C method swizzling and class-dump to see what to patch. SIMBL (http://www.culater.net/software/SIMBL/SIMBL.php) uses the same method of injecting based on custom Apple script.