You can write complete apps for almost any smartphone platform (Android, iOS,...) using Phonegap. (http://www.phonegap.com)

It is an open source framework that exposes native capabilities to a web view, so that you can do anything a native app can do.

This is very suitable for cross platform development if you're not building something that has to be pixel perfect in every way, or is very hardware intensive.

If you are looking for UI Frameworks that can be used to build such apps, there is a wide range of different libraries. (Like Sencha, jQuery mobile, ...)

And to be a little biased, there is something I built as well: http://www.m-gwt.com