Collapse-O-Matic is a plugin that I have used on more than once occasion already. It works straight out of the box and is highly customizable for anyone that wants to play around with it and/or push it to the limits :)
On a WooThemes theme I have just combined the title with a button (by adding the button class to
trigclass) and the result is so cool!
I can highly recommend the plugin as well as the author Baden who is very active with supporting his plugin!