of Applied Cryptography
The best reference book on cryptography. The whole book can be read online.
Some must have utilities, many of them freeware, including RootkitRevealer, TCPView and NTRecover.
Some useful security tools are available here including scanning tools and forensic tools.
Gibson Research Corporation
Some usefull security utilities.
Provides an encyclopaedia of IT terms.
Portal site on IT Security, hosting the famous Bugtraq mailinglist,
where vulnerabilities are posted as they are discovered.