Adobe® AIR® for Android
The Adobe® AIR® enables developers to use Adobe Flash® Professional software, and ActionScript® to build web applications that run as standalone client applications without the constraints of a browser.
Android SDK by Google Inc.
The Android SDK provides the tools and APIs necessary to begin developing applications on the Android platform using the Java programming language.
Corona® SDK by Ansca® Mobile
Ansca® Mobile is the maker of Corona® SDK - a mobile development framework that lets developers build applications quickly for iOS and Android. Corona® SDK has built-in support for 3rd party services, like Facebook, Openfeint, and monetization.
Mono for Android
Mono for Android enables developers to use Microsoft™ Visual Studio™ to create C# and .NET based applications that run on Android phones and tablets. Developers can use their existing skills and reuse code and libraries that have been built with .NET, while taking advantage of native Android APIs.
PhoneGap is an HTML5 app platform that allows you to author native applications with web technologies and get access to APIs and app stores.
UNITY: Android
Unity 3 is a game development tool that has been designed to let you focus on creating amazing games. Unity gives you the power to painlessly develop applications for any of the 30 million-plus Android devices out there.