RubyMotion!

PhoneGap is an open source development tool for building iPhone, Android, Blackberry and other mobile apps with JavaScript.

Apache Cordova is a set of device APIs that allow a mobile app developer to access native device function such as the camera or accelerometer from JavaScript. Combined with a UI framework such as jQuery Mobile or Dojo Mobile or Sencha Touch, this allows a smartphone app to be developed with just HTML, CSS, and JavaScript.

Xcode is Apple's premiere development environment for Mac OS X. In addition to being packaged on the DVD with every copy of Mac OS X, the latest Xcode developer release is always available for free to ADC members and includes all the tools you need to create, debug, and optimize your applications. At the heart of the Xcode tools package is the Xcode IDE, a graphical workbench that tightly integrates a professional text editor, a robust build system, a debugger, and the powerful GCC compiler capable of targeting Intel and PowerPC regardless of host platform. Xcode is both easy to use, and yet powerful enough to build the largest Mac OS X applications. The complete Mac OS X developer tools chain is distributed as part of Xcode; these tools include Interface Builder, Instruments, Dashcode, the WebObjects framework, and the complete reference documentation, to name just a few. »

Type intelligentlyWorld-class code completion unlocks the iOS, Android, and OS X APIs. Quickly look up methods and types, and discover API functionality.

Appcelerator Titanium, offered for free by Appcelerator, lets web developers use web technologies, open source and cloud computing to build native apps for iPhone, Android, Windows, Mac OSX and Linux.

WebDGap allows you to convert any website or HTML/CSS/JavaScript web application to a native Windows, Mac, Linux, and Chrome application.

Rapid Application Development tools for native Android, iOS and desktop applications. The simple way to quickly program real world apps. Programming language similar to Visual Basic.

Corona has become one of the world's most advanced mobile app development tools.

Codename One provides a comprehensive solution for WORA (write-once-run-anywhere) mobile app development. You write the applications in Java using and deploy the application to Android, iOS, Windows Phone, or other platforms. The resulting app is a native app (not HTML5).

MonoTouch allows developers to create C# and .NET based applications that run on the iPhone. It is based on the Mono framework and developed in conjunction with Novell. Unlike Mono applications MonoTouch "Apps" are compiled down to machine code targeted specifically at the Apple iPhone. This is necessary because the iPhone kernel prevents just-in-time compilers from executing on the device.

nativeCSS is a free framework to make app development easier. Use CSS styling in your apps today.