Recently I tried developing an application for Facebook as my client demanded. Facebook uses XFBML (eXtensible FaceBook Markup Language) as their markup language to use Facebook UI and API. And it took me a while to test and get it right since I had to try and error it for every change I made. And I just found (I know it has been around a while) the Facebook FBML live testing tool provided by Facebook developers.
This application is called Rell. It is an FBML and JavaScript test console that saved so much of my time. I just need to use the example code and modify it to my own need, then test it out. If it is good then I will implement it in my code. Then finally test it live from the Facebook iframe.
You can find the Facebook FBML live testing tool at:
Facebook JavaScript Test Console
and
FBrell
Both links are the same thing. You can use whichever you want. I prefer the FBrell one, since it is full screen and less distracting, so I can concentrate just on my code and the result.