I needed a lightweight JavaScript feed parser based on jQuery and couldn't find one, so I wrote jFeed yesterday.
Thanks to jQuery, it was quite easy and fun to do.

jFeed currently parses RSS 0.91, 0.92, 1.0, 2.0 and Atom 1.0 feeds.

Usage:

jQuery.getFeed(options);
   
   options:
   
   * url: the feed URL (required).
   * data: data to be sent to the server. See jQuery.ajax data property.
   * success: a function to be called if the request succeeds.
     The function gets passed one argument: the JFeed object.
   
   Example:
   
   jQuery.getFeed({
       url: 'rss.xml',
       success: function(feed) {
           alert(feed.title);
       }
   });

JFeed properties:

   * feed.type
   * feed.version
   * feed.title
   * feed.link
   * feed.description
   * feed.language
   * feed.updated
   * feed.items: an array of JFeedItem

JFeedItem properties:

   * item.title
   * item.link
   * item.description
   * item.updated
   * item.id

jFeed is dual licensed under MIT/GPL.
Download jFeed

Examples are provided in the archive, as well as a basic PHP proxy (testing purposes only) for loading external feeds.

Note: the feed must be sent as XML, thus the content-type needs to be specified as text/xml or application/xml, see Specifying the Data Type for AJAX Requests.