A standalone version of gdx-tools' DistanceFieldGenerator.
See the Releases section for runnable JARs.
See the libGDX wiki for more information.
You can run the JAR with a command like the following, changing downscale and spread as the wiki describes:
java -jar DistanceFieldGen-1.11.0.0.jar input.png output.png --downscale 8 --spread 64
This was taken from inside libGDX and moved so it can be run more easily from outside that framework. It still has the same license as libGDX.