Mobile - Best UI for Cross Platform Mobile Development

jQuery Mobile

Touch-Optimized Web Framework for Smartphones & Tablets.
A unified, HTML5-based user interface system for all popular mobile device platforms, built on the rock-solid jQuery and jQuery UI foundation. Its lightweight code is built with progressive enhancement, and has a flexible, easily themeable design.

Seriously cross-platform with HTML5 

jQuery mobile framework takes the "write less, do more" mantra to the next level: Instead of writing unique apps for each mobile device or OS, the jQuery mobile framework allows you to design a single highly-branded web site or application that will work on all popular smartphone, tablet, and desktop platforms.

• iOS, Android, BlackBerry™, Bada, Windows® Phone, Palm webOS™, Symbian, MeeGo™

GitHub:
https://github.com/jquery/jquery-mobile

Online Demo
http://jquerymobile.com/

W2UI

Overview
Web 2.0 Touch is a concise JavaScript library for touch devices (iPhone, iPad, Android) or anything that runs on WebKit engine. Below is a short list of features.

• HTML5 and CSS3
• Dynamic page loads
• Hardware accelerated transitions
• Several CSS themes
• EdgeToEdge and Rounded lists
• Native looking tabs
• Native looking buttons
• Regular and Modal Overlays
• Much more...

GitHub
https://github.com/vitmalina/Web-2.0-Touch

Online Demo
http://w2ui.com/demo/touch - only WebKit browsers