Matthias Brutella created the ChatHeads open source project to mimic the functionality found in the Facebook app, where avatars of your friends are shown in small bubbles. The bubbles can be dragged around on the screen and when pressed, a conversation is started.
In Matthias’s words: The goal of this project was to implement the interaction (drag and release, pressing) with the ChatHead bubbles as well as the animations.
The project includes some interesting code, including unique animations. Read the info on github where Matthias explains the math behind the bounce animation.
You can download ChatHeads from github.