Finding Web Vulnerabilities (a primer)
charleshooper.net
charleshooper.net
If you found the author's post interesting and would like to see some practical examples of how to test for application security vulnerabilities, then the following will help get you up to speed:
http://www.owasp.org/index.php/Category:OWASP_Testing_Projec...
http://www.owasp.org/index.php/Category:OWASP_Code_Review_Pr...
Edit: Here we go:
The testing guide: http://www.owasp.org/images/5/56/OWASP_Testing_Guide_v3.pdf
The code review stuff: http://www.lulu.com/content/5678680