Appcelerator would be a good choice for non-game applications, it uses native UI elements as opposed to running apps in the mobile browser, lets you code in JavaScript and deploy to Droid, iPhone, iPad and soon Blackberry from one project.
For games I'd take a look at Corona. This lets you develop for Droid and iPhone in Lua which is even easier than JavaScript. They recently released a beta Windows SDK (it was previously Mac-only). This is a great SDK but lacks in controls that you would need for non-game applications.