Titanium Developer is a cross-platform IDE that offers rapid application development for iOS (iPhone, iPad, iPod Touch, etc), Android, and the desktop. It is made by Appcelerator, Inc.
The Community Edition is free and open-source. There are also Professional and Enterprise versions.
Essentially, using TD, when you want to create an app, instead of writing it in Objective-C (as you normally would for iOS) or in Java (as you normally would for Android), you just write a single code base in Javascript, using TD's API, and TD will then compile your code to NATIVE code for the platform(s) you specify. It uses the official SDKs for those respective platforms to do this, so you will need to download and install those before installing TD.