ExtJS... something you should add to your developments.

Ext JS is a cross-browser JavaScript library for building rich internet applications.

• High performance, customizable UI widgets
• Well designed, documented and extensible Component model
• Commercial and Open Source licenses available

I ran into this a while back doing a project (thx Paul and Nick for pointing this out), and have been keeping an eye out on their latest developments. Version 2.1 now available for download [2] allow you to enrich your web application in ways you can only dream of. You don't believe me, check it out for yourself here [3].