Cross platform GUI designer for custom file format

I have a hobby project going on these days where I have made my own file format for the GUI, but it is very tiresome to edit text files all day. So I was wondering if anyone knew of any tool for designing UI which is scriptable or maybe support plug-ins so I can make a it save the UI to my own file format. I know about a couple of tools my self, but I want more suggestions before I make up my mind.

Catch 1: It has to be free for non-commercial use.
Catch 2: It has to be cross platform.
Catch 3: It is cold outside, although that is not relevant.

[Edit] I forgot to emphasize on what kind of tool. It can be what ever like an IDE, Drawing/Painting tool, or a tool that is made specifically for this purpose.


- ØØ -