This format is used in many game frameworks, like libgdx and several Unity3D plugins, and probably many many other ones.
Most tools available allow you to generate these descriptors by rasterizing true type fonts (like Hiero or Bitmap Font Generator), but I couldn’t find any simple way to quickly generate this file, when you start with an existing bitmap sheet.
How to use
It’s really straightforward. First, click on “open an image”. Try this one if you don’t have any yet.
Tweak the char width and height until the grid match perfectly the letters in the image (8×8 for the example I linked). Set the lines numbers (7), then manually write down the glyphs in the inputs below.
Click on “preview” to check that everything (glyph mapping and cutting) is fine. Once you’re satisfied with the result, simply click on “ouput”, and copy paste the result in a new fnt file.
If you have any question, or suggestion, feel free to comment! Alternatively, you can even contribute yourself to the code.