I've been playing with Facebook for a little over a week and I couldn't resist trying out the Facebook Platform. To be honest despite the missing documentation and changing APIs, they've done a nice job.
My first attempt at an application took a little over 3 hours and is called Easter Egg. It lets you leave Easter Eggs on your profile that only certain visitors can see. So, for example, you could leave a little love note for your girlfriend, or details of a secret party for your buddies, or just say Hi! when your Mom visits. And here's a screen shot:
If you are a Facebook user and like that sort of thing you can add Easter Egg.