There's a lot of tools out there to help aspiring developers get started on a new Android app, not the least of which is the newly-resurrected App Inventor from MIT. Now would-be app makers have a new option, specially suited to those whose skills or inclination tend towards Java: AIDE. The free app stands for "Android Integrated Design Environment", and is a full-fledged Java IDE contained within an Android app.
Speaking basically, AIDE is a series of pre-compiled code sections combined with a text editor, compiler and tester. Putt all those functions together and you get A contained system for editing and creating Android apps. You can even compile the APK files right on your phone, literally crating an app with another app. This sort of thing happens all the time with more season desktop operating systems, but to my knowledge, it's a first on a mobile platform.
It's not quite as simple as the App Inventor: you'll need basic Java skills to create anything more complicated than a test app. For those who need it, AIDE is compatible with the Eclipse IDE, and it admirably adjusts its interface for smartphones and tablets. Amazingly, the developers are giving away the app for free. Download it form the Android Market here.