I hacked together a Facebook application recently. I was looking in to integrating Becontrary.com with Facebook, but I had an idea for a simple app that would help me learn the ropes with FB. Its called Virtual Microbes -- the idea is that you give your friends a virtual disease and encourage them to pass it on. You gain a point each time you give a friend a microbe and each time a friend (or friend of friend) passes it on, which has the potential for exponential growth. Its completely pointless, and couldn't exist outside of the social networking sandbox, but it gave me the opportunity to experiment with integrating Turbogears with Facebook's application system. There was some pain initially, but ultimately it went smoothly. I'll write up my experiences with it, and give away some of the code soon.
I look forward to seeing how it goes.