We've built an API Test Console that allows you to interactively experiment with the methods available in the Facebook API. Now you can see the functionality the API makes available without writing a single line of code.
To use the console, select the method you wish to call and supply any required parameters. You'll be able to see the response just as you would if you were writing your own app. We even provide two response formats: XML and PHP. This way, you can simulate using the Facebook PHP Client Library. Be sure to be logged into Facebook before trying out the Test Console.
One caveat is that identifiers used in the console won't work in normal apps. This means that your user ID displayed in the test console will be invalid in your own app (and vice-versa - the console will reject IDs from other apps as well).
We're hoping that allowing you to play around with the API and directly view the format of method responses will make it easier to get started building your own app. Give it a try.