Best ajax library for Sharepoint

For developing Windows Sharepoint Service application, what is the best ajax library available now? I'm thinking of free, customizable, and easy deployment, and its compatible with sharepoint, meaning it can run in 'quirks' mode.

From stackoverflow Fajar

• RadControls by telerik is pretty good.

  edit: wtf, stole my answer

  From Shawn Simon

• If by WSS application you mean a SharePoint or MOSS site, there are no free full-featured AJAX libraries that are specifically designed for MOSS.

  However, the most recent service pack for SharePoint 2007 included some updates to improve compatibility with the standard ASP.NET AJAX control suite.

  Telerik's RadControls are far from free (1000USD per developer seat), but their RichText editor has a MOSS-specific version. Microsoft's official guidance for enterprise deployments is to use the Telerik RadEdit control instead of SharePoint's built-in rich text editor. I can say from experience that it is not bug-free, but if you just plan on using it out of the box, it is pretty slick.

  As far as the rest of Telerik's AJAX controls, implementation inside of MOSS is difficult but possible.

  From Rex M