Cool! It looks nice.
Padding is extra pixels around glyphs for effects? What is spacing? Seems to do the same, so I think you only need padding.
What about x-advance and y-advance adjustment? Often you set the padding to make glyphs bigger so you can have effects, then set the advance to the inverse of the padding, so the padding doesn't affect the distance between glyphs when text is drawn. Sometimes you need a pixel or two extra in the advance so effects don't overlap neighboring glyphs.
It uses the guillotine (aka black pawn) packing algorithm? I wonder the difference between TexturePacker2's maxrects+brute force algorithm. TexturePacker2 can also compute the smallest size image that fits all regions, even for non-POT.
Does it have kerning?
C:\windows\fonts doesn't show any fonts in the FileDialog, I guess because it isn't a real directory. A list of system fonts would be nice. For Sun VMs you can get the font file with some reflection (see Hiero or my recent JGO post about it). Maybe you could only show the system fonts if the reflection hack succeeds.
Test fonts screen is nice. Could scroll.
People might want to have multiple fonts with different size for each. Oh, I see you have a Java API for this, nice!