Depends on your requirements. JQuery is great if your goal is also to maintain a progressive experience that somewhat works without javascript.
If you're after a feel closer to desktop apps, or a high level of interactivity, and also don't care much about users who don't sport a modern browser, I think the best library is Ext, both in terms of community size and widget features. But, qooxdoo would be a better choice among free alternatives, (i.e. Sproutcore and Cappuccino)
In a nutshell: In case of Sproutcore, I don't think Sproutcore's widget set is great. As for Cappuccino, I'm not sure about running code over an "Objective-j" layer on top of javascript, which is slow enough. And, I'm not a mac guy at all.
Qooxdoo has introduced some support for progressive pages as well, but it's nowhere near JQuery. I'm also maintaining a project called JsQt, that converts .ui files from Qt Designer to qooxdoo code. See http://to./3t6p to get a quick feeling of what works and what doesn't. So you also have a basic gui-designer support for qooxdoo as well.
Ext looks great, but for some reason, I've never felt like their licensing seemed very clear cut. Having said that, I'd consider Ext over the likes of Sproutcore or Cappuccino.
In cappuccino you compile the objective-j to javascript for release so the speed issue doesn't exist. It is a new syntax for most javscript devs though so that might be a turn off.
are you absolutely sure? here's an excerpt from their website:
"""
Cappuccino was implemented using a new programming language called Objective-J, which is modelled after Objective-C and built entirely on top of JavaScript. Programs written in Objective-J are interpreted in the client, so no compilation or plugins are required. Objective-J is released alongside Cappuccino in this project and under the LGPL.
"""
I'm developing an objective-j app right now and they have compile targets that compile directly to minified javascript. It will work without compiling but for a production I wouldn't recommend it for bandwidth reasons alone. Technically they are right the javascript is still interpreted in the client and you don't need any browser plugins to run it. It's not strictly objective-j source code your are running in a production build so there aren't too interpreter passes.
The Debug build targets don't compile to javascript though so there are two interpreter passes in that case. That is fairly speedy and I don't notice an insane startup difference.
Actually, the main difference between debug and release are that release builds also minify/compress the code. Both compile from objj to js.
Of course, you can not run the compiler at all and the code will still run in your browser, because it will do the compilation "just in time". There's not a significant performance penalty for doing this, and browser refreshes are still speedy. But when you're ready to deploy, you might as well build ahead of time even for a 10-20% launch time speedup.
If you're after a feel closer to desktop apps, or a high level of interactivity, and also don't care much about users who don't sport a modern browser, I think the best library is Ext, both in terms of community size and widget features. But, qooxdoo would be a better choice among free alternatives, (i.e. Sproutcore and Cappuccino)
In a nutshell: In case of Sproutcore, I don't think Sproutcore's widget set is great. As for Cappuccino, I'm not sure about running code over an "Objective-j" layer on top of javascript, which is slow enough. And, I'm not a mac guy at all.
Qooxdoo has introduced some support for progressive pages as well, but it's nowhere near JQuery. I'm also maintaining a project called JsQt, that converts .ui files from Qt Designer to qooxdoo code. See http://to./3t6p to get a quick feeling of what works and what doesn't. So you also have a basic gui-designer support for qooxdoo as well.