Beginner
Try jQuery (try.jquery.com)
jQuery (www.codecademy.com)
Treehouse - Build an Interactive Website (teamtreehouse.com)
Learn X in Y Minutes - jQuery (learnxinyminutes.com)
What is the DOM? (css-tricks.com)
Event Handling (eloquentjavascript.net)
What is AJAX? (www.youtube.com)
AJAX with jQuery (www.elated.com)
JSON: What It Is, How It Works, & How to Use It (www.copterlabs.com)
jQuery Best Practices (gregfranko.com)
Intermediate
Advanced