PhoneGap is a free and open-source framework based on Apache Cordova. You can develop mobile apps for 8 different platforms including Android and iOS. PhoneGap let’s you develop mobile apps using web technologies and has been around for some time now earning developer’s trust.
Xamarin is a cross platform tool to develop mobile apps in C#. Apps built with Xamarin are native which means user interfaces look and feel like native apps coupled with native grade performance. It offers a vast range of native APIs and supports the Android, iOS and Windows Phone platforms.
RhoMobile is a set of tools for cross platform app development using web skills. It supports Android, iOS and Windows Phone app development in Eclipse using a powerful plugin. RhoMobile is specifically targeted for enterprise users because it can easily integrate and synchronize with enterprise backend systems like SAP and Oracle.
MoSync is a free and open-source set of tools for building cross platform apps. The SDK lets you develop apps using C/C++ or HTML5 and deploy then on 9 different platforms.