Appcelerator Titanium is one of the top choices for building cross-platform mobile apps. You can develop native iPhone/iPad, Android, and mobile web (and soon BlackBerry) apps with full access to each device's API. Applications built with Titanium are social, local, media rich, interactive, and extensible and support all of the iPhone, iPad and Android UI, including table views, scroll views, native buttons, switches, tabs, popovers and more.
Adobe's (formerly Nitobi's) PhoneGap -- widely recognized as a game-changer for mobile app development -- is an open source mobile development framework for building powerful cross-platform mobile applications.
Motorola's RhoMobile offers a very impressive suite of polished tools -- Rhodes framework, RhoSync server, RhoHub Development as a Service (DaaS), and RhoGallery app store -- to quickly build and deploy powerful native mobile applications for all smartphones: iPhone, Android, Windows Mobile, BlackBerry, Symbian and tablet devices such as iPad & Samsung Galaxy Tab. These native device apps work with synchronized local data and take advantage of device capabilities such as GPS, PIM contacts, camera, native mapping, push, alerts and calendar.