Recently i tried to developing an application for Facebook as my client demand. Facebook use XFBML (eXtensible FaceBook Markup Language) as their mark up language to use Facebook UI and API. And it took me a while to testing and do it right since i have to try and error it for every changes i made. And i just found (i know it’s been a while there) Facebook FBML live testing tools provide by Facebook developer.
The both link are the same, you can use whichever you want. I prefer use the FBrell one, since it is full screen and less distraction, so i can concentrate just to my code and the result.