ForceCertified is a free resource for Force.com Certification by John Coppedge. http://forcecertified.com

ForcePrepare is a good site for preparation on the Certification exam. It also has a mock test to solve. http://www.forceprepare.com/

This small guide contains tips about the Force.com exam. http://mysalesforcecode.blogspot.com/2009/03/developer-401-forcecom-exam-tips.html

The iTunes podcast is a free and the most important of the materials available online. It contains videos of the DEV401 classroom trainings. http://itunes.apple.com/podcast/-training-certification/id325668840

The SalesforceChannel hosts all the videos about Salesforce, and Webinars. They have a good section on Developer Training videos. http://salesforcechannel.com/pages/developer-training