"The Open Web Application Security Project (OWASP) is dedicated to finding and fighting the causes of insecure software. Everything here is free and open source. The OWASP Foundation is a 501c3 not-for-profit charitable organization that ensures the ongoing availability and support for our work. Participation in OWASP is free and open to all."
A Wiki dedicated to discussion of web application security. This site covers everything from papers, HOWTO's, and downloads to information on conventions and blogs.
Certainly a useful resource for those who host, write, or manage web applications.