Mug-android is a project mainly written in Java, it's free.
JavaScript on Android.
Mug-Android is an Eclipse project, developed with Android plug-ins. It might work with vanilla Android tools.
Download the source, load it as a new Eclipse project. You should first run "ant build" in the root directory to generate lib/app-js.jar, which is converted by Dex into Android native code.
JavaScripts are in assets/scripts/. All JavaScript files are (re)compiled when running the Ant script. You can have multiple scripts in the folder and require them, though main.js is required.
At the moment, main.js expects a getMainActivity export. Currently working on how to define new activities in a scriptable manner.