One weekend a few months ago, I decided to try my hand at creating a Facebook app. The premise was simple: create a game where you could prepare for the inevitable Zombie Apocalypse by choosing a team from your friends, specifying how exactly they'd be useful in the event of certain doom.
Heroku has excellent Facebook integration, and it wasn't long before I had a basic app up and running. Part of the game involves a Facebook friend selector, and after searching around for some existing libraries, I decided to write my own one using Spine.
The picker allows you to toggle the selection of multiple friends, and fires events whenever the selection changes. You can even filter by name. It's a good example of how simply this sort of behavior can be represented with client-side MVC.