A standalone version of gdx-tools' ETC1Compressor.
See the Releases section for runnable JARs.
This should be on Maven Central now; you can depend on it with Gradle:
api "com.github.tommyettinger:libgdx-etc1compressor:1.11.0.0"
or if you use Maven (unlikely unless you know you need it):
<dependency>
<groupId>com.github.tommyettinger</groupId>
<artifactId>libgdx-etc1compressor</artifactId>
<version>1.11.0.0</version>
</dependency>
See the libGDX wiki for more information.
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.