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 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.