Background: I have only recently begun programming GUIs in Mathematica, so I have a continuous need for custom controls. I needed a scrollable list control and with the help of Google I found a beautiful one in this MathGroup post (One of Sjoerd, if I am correct.) The point being: there is a lot of excellent Mathematica code shattered all over. Most matured languages have libraries of custom controls. I am not aware of any existing for Mathematica.
Question(s): What is the best managed collection of (open source) custom GUI controls for Mathematica that you know of? Where do you get your custom GUI controls? Should there be a collection of custom GUI controls for Mathematica?