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:
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.