OWASP (Score 2, Informative)

Your first stop should be OWASP, the Open Web Application Security Project. You'll find there many companies that are experts in web application security, including tools and guides to get a handle on web app sec. I'd also recommend becoming familiar with the OWASP Top 10 http://www.owasp.org/index.php/Category:OWASP_Top_Ten_Project