I find the Android Scripting Environment to be quite cool:
<a href="http://code.google.com/p/android-scripting/" rel="nofollow">http://code.google.com/p/android-scripting/</a><p>And it offers quite a few languages to play with, including Python, JRuby and Javascript (via Rhino).<p>Now if it would only have a better development environment...