Extending classes

[eluser]stuffradio[/eluser]
I stupidly hadn't really looked at extending classes until today, but now I realize what I could do. I can use something like My_Controller to use with a permission system, and I'm sure lots more things.

What are the most common classes you guys extend and what do you do to make your code the most efficient and most reusable so you don't have to write things over again?