OWASP (Open Web Application Security Project) http://www.owasp.org/
OSSTMM (Open Source Security Testing Methodology Manual) http://www.osstmm.org
The NIST SP 800-42 at: http://csrc.nist.gov/publications/nistpubs/800-42/NIST-SP800-42.pdf
The CISSP and SSCP Open Study Guides Web Site http://www.cccure.org
The Professional Security Testers Warehouse http://www.professionalsecuritytesters.org