Always wanted to try out creating a Facebook  Application and finally took a little bit of time of and created a simple Facebook App called GaMerz.WordPress.
The application simply displays my live site traffic (users online) and the latest post for this site. The file is placed in the WordPress plugins folder but it does not need to get activated. If anybody wants the source code, just drop me an email.
GaMerZ.WordPress Facebook Application
Here are some links to help you get started:
– Facebook PHP Client Library 
– Step-by-step Guide to Creating a Facebook Application 
– Facebook Developers Wiki