Various CDN's Around the Web

Obviously Google's CDN is what you want to use, as it is another link into the google matrix that helps your page ranks.

Google CDN Dev Guide

Here is a link to add the latest v1.x.x version of jQuery to your page hosted by Google-

http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js

There are alternatives though, you can you CDN libraries hosted by Microsoft-

Microsoft Hosted CDN's

And there is the new kid on the block,

cachedcommons.org

Cached commons looks promising as it host A LOT of javascript libraries, not just jQuery.

I'll add more links as I find them.