Alan Levine’s 50 Tools for Online Storytelling is, of course, a good place to start if you are searching for free, easy-to-use online tool.
Are you looking for media to use? (Please add to this list!) In addition to the resources mentioned by Alan Levine, and that you are well aware of (Youtube & Flickr, for example), you might find the following helpful:
Archive.org for image, audio, video.
Freesound for all kinds of audio, including sound effects.
Jamendo for free, copyright-free music!
Zamzar for free file conversion.
Typogenerator for random effects with your text, such as:
How-tos
The Center for Digital Storytelling’s Cookbook (It includes a storyboard).
Professor Wesch’s How -to Tips on Making Youtube Mash-ups
Stories for Change website contains a wealth of resources.
J-Term Contemporary Creative Nonfiction 
Does anyone know how to search for only non-copyrighted images on Flickr? Thanks!
Miriam
Sure. Click on search, and then do an Advanced Search. Make sure you’ve clicked the button that says ‘only find creative-commons licensed pics’ and voila!