Best of breed facebook applications?

Hi everyone,

I have a rather broad question - I am looking for your thoughts on what you
think are some examples of well designed / well executed facebook
applications, especially those that focus around allowing users to message
each other? I'm mostly interested in seeing examples of apps that really
excel in the user interaction and presentation. Too many apps feel jarringly
out-of-sync with the overall "facebook experience".

I'm getting set up to design a facebook app for the first time, and I'm just
looking around for what people better than me have done already...

Happy Friday,
Taneem