Lion-Scrollbar is a project mainly written in OBJECTIVE-C and C, it's free.
This repository brings the new Lion scroll bars to Snow Leopard (OS X 10.6). The Xcode project contains two classes:
You can use the Lion Scroll View on a per scroll view basis. Simply select the scroll view in Interface Builder/Xcode and change the class from NSScrollView to CMKScrollView. Do the same with the scrollers embedded in the scroll view. Done.
The Xcode project contains an example application.