1. App Inventor
What is it: A toolkit enabling you to quickly make simple Android apps.App Inventor was originally designed as an educational programme to teach children the basics of programming apps. It quickly became adopted by a user community that expanded its capabilities so that more sophisticated Android apps can be made with it - in fact, some apps made with App Inventor are available in the Android Market. Google actually announced their decision to cease further official development on App Inventor, but they have assured the project will be released to its community as an open-source project.