From Philippe Mougin:
F-Script is an award winning set of open source tools that complement Xcode and Interface Builder to provide scripting and interactive access to Cocoa frameworks and custom Objective-C objects. It aims to be a useful and fun tool for both beginners and experts, allowing interactively exploring, testing, and using Cocoa-based objects and frameworks.
F-Script can be embedded in application thanks to a set of easy to use components. It can also be dynamically injected into any Cocoa application, providing a powerful environment for exploring or manipulating applications from the inside.
The main new feature in this version is support for adding methods to existing classes. This is similar to Objective-C categories, but works dynamically at run-time. This is quite useful for prototyping stuff and experimenting with new designs, as well as for making object-oriented frameworks developed independently work together. Besides, it is a natural extension of the ability to define Cocoa classes directly in F-Script and lets you write a bigger part of your application code in F-Script, should you want to.