MooTools continues to impress me. Check out the Class.implement and Class.extend functionality in this great article from Cory Hudson (there are numerous code examples).

mootools implements a class inheritance scheme that is inspired by Dean Edwards’ wonderful Base class. Creating a class is similar to Prototype, but now you don’t ever have to think about the prototype object when you define the class…

…mootools supports multiple inheritance in a Ruby mixin style with the Class.implement() method. Using the implement method of an existing class, that class can inherit methods and properties from another class or object. Unlike Class.extend(), the implement method is called on the class you are modifying, not on the object being copied. This method can be used to support multiple inheritance.

And here’s another article from Cory on MooTools numerous helpful utility functions:

The new mootools JavaScript framework has quickly impressed me with its design and usefulness. The library was clearly written to meet real programmers’ needs while working in JavaScript. Just take a look at some of the new utility functions and methods it provides.

Note: This article covers functions and methods found in the Array.js and Function.js modules of mootools.