Scroll bars are one of the most subtle parts of an operating system's user interface, but also one of the most used. Sure, you probably use your touchpad or mouse to scroll around web pages and documents, but these slight tools, usually fixed to the side of the program, are much needed. In the past they allowed you quickly scroll up and down pages, whereas now-a-days they act as mere indicators of where you on a page.
Ivan Lists posted this cool graphic that shows how much the scroll bar has evolved since the very first graphical user interface, the Xenox Star, to Mac OS X Lion.
I'm partial to the scroll bar from the original Mac OS X.