Microsoft Security Site (all audiences)

http://www.microsoft.com/security

MSDN Security Site (developers)

http://msdn.microsoft.com/security

Common Weakness Enumeration (CWE) (developers)

http://cwe.mitre.org/top25/index.html

Most Common Software Errors

http://discussweb.com/software-testing/803-most-common-software-errors.html