Recently found out about a couple cool Facebook projects worth checking out built on the Facebook Toolkit (which is the #4 most downloaded project on Codeplex of over 3000 projects):
Synchronizer 1.0 by Lutz Roeder - Synchronizer is a Facebook desktop application that allows you to synchronize profile pictures, birthdays and work info with your Outlook contact list. It also allows you to create Outlook contact items for existing Facebook friends and matches them if named differently.
Facebook Data Store Client API - Facebook has a beta distributed Data Store accessible through REST web services. There is already a provisional PHP client API in existence, but there is no C# API - until now!