Calligro

Calligro lets you generate custom fonts from images created in graphics software like Gimp, Photoshop, Aseprite and others.

The workflow is as follows:

• generate a Calligro template (which is a .png image)
• draw your characters on the template in any graphics editor
• load the filled template back into Calligro and save a bitmap font

This tool is open source. You can find the source code on GitHub. We also maintain a web version at calligro.ideasalmanac.com. It has the same core feature but the offline version is a bit more convenient. 

Calligro generates fonts in the BMFont format. A lot of game engines and frameworks support it out of the box. This includes Godot, Phaser.js, LibGDX, Heaps.io, HaxeFlixel and others.

You can also generate non-monospace fonts but only Godot seems to support it.

Be sure to check out the official tutorial