Open Web Application Security Project
"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."
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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.
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