Some tutorials for Web Sites Usability:
1. Professional Website Usability:
http://www.sitepoint.com/article/223
2. Jesset Usability Tutorial:
http://www.jessett.com/web_sites/usability/index.shtml
3. Principles and strategies for practitioners
designing universally usable sites:
http://www.otal.umd.edu/UUPractice/
4. Criteria for optimal web design (designing for usability):
http://psychology.wichita.edu/optimalweb/