[sorted alphabetically]
Popular site's general Internet/network security info. 
Federally funded security R&D center at Carnegie Mellon University in Pittsburgh. 
Various security topics & white papers from a leading info security company. 
Computer Security Institute (CSI) serves the needs of security pros worldwide. 
Common Vulnerabilities and Exposures - dictionary of known security vulnerabilities. 
Security news for enterprise IT and network security pros (Opening ad?). 
Foundstone - site founded by security experts offering many services (McAfee owned). 
National Institute of Standards & Technology - National Vulnerability DB 
SANS - Largest source for info security training, certification & research in the world. 
Security-specific info resource for enterprise IT pros (opening ad?). 
Webroot Computer Security Center