The Open Handset Alliance, a group of more than 30 technology and mobile companies, is developing Android which is the first complete, open, and free mobile platform. It is a software stack for mobile devices that includes an operating system, middleware and key applications. This beta version of the Android SDK provides the tools and APIs necessary to begin developing applications on the Android platform using the Java programming language. They’re offering the Android Software Development Kit as well. There are four major features Android provides.

Open
Android allows you to access core mobile device functionality through standard API calls.

All applications are equal
Android does not differentiate between the phone’s basic and third-party applications, even the dialer or home screen can be replaced.

Breaking down boundaries
Combine information from the web with data on the phone, such as contacts or geographic location to create new user experiences.

Fast & easy development
The SDK contains what you need to build and run Android applications, including a true device emulator and advanced debugging tools.

Requirements: -
Demo: http://code.google.com/android/
License: Apache License 2.0