Googles App Inventor is a visual(html + java webstart) IDE to develop apps. using drag and drop mechanims.

Google App Inventor is currently - not as flexible/powerful as developing using Eclipse/java, but it can be useful for mockups/small customization . In future you may develop the components using java.

You can find a quick tutorial/overview here:

http://android-java.blogspot.com/2010/09/getting-started-with-app-inventor.html