The payoff is in size. The current version of Zepto weighs in at about 2,300 bytes, about one-tenth the size of jQuery. XUI is just a bit bigger. The comparison to jQuery is apt because both borrow much of the syntax without offering any of the features that require more sophisticated and heavy code. XUI, for instance, pulls out the code that would be required to be compatible with BlackBerry or IE Mobile. If you want them, you can get a fatter library.
Mobile Web programing tool: Jo and Sencha Touch
Download Sensa Touch
Sencha Touch is built by a burgeoning company, Sencha, that also offers a framework for regular Web pages. It sells a collection of support plans but does not charge for a commercial license to use Sencha Touch. The company's development team answers questions for users and maintains an open support forum.
Jo is an open source project delivered with the OpenBSD license. It is free to use, and Dave Balmer, Jo's developer, runs support for those who need help.
I've built several Web applications with Sencha Touch and have found the process relatively easy because the framework handles many of the layout questions. In the best cases, I simply created a new widget object, and the Touch framework would squeeze it into to the page so that it looked nice. Some of the resulting apps worked well on both the iPad and the iPhone despite the different screen sizes.
Both Jo and Sencha Touch are producing more code than documentation right now. Each should be filling these gaps soon, but for the time being, Sencha's commercial support offers the deeper documentation of the two.
Mobile Web programing tool: jQTouch
jQTouch was one of the first great frameworks for mobile Web applications. Its creator, David Kaneda, has since left the project to work for Sencha, but Jonathan Stark has taken the mantle and continues to add worthwhile tweaks to the code.
Applications in jQTouch are built by inserting HTML in DIVs. jQTouch parses these DIVs looking for the right classes, then inserts its own code for handling events.
I've built several Web applications on top of jQTouch and found it's as simple as creating a Web page. It's also a bit easier to integrate with dynamic Web tools like JSPs, PHP, and other server-based frameworks.
In the right situations, the code looks identical to native apps. But sometimes I've found odd glitches and weird transformations that don't make sense. Some of the touch events are also a bit slow on certain platforms.
Mobile Web programing tool: PhoneGap