Alec Muffett's Home Page - The author of Crack, a Unix password cracker. A collection of software, libraries, and documentation for Unix security.

Fyodor's Exploit world - Nice archive of exploits for misc systems.

Packet Storm - Packet Storm is the largest collection of computer security information on the web. All the newest tools, exploits, and white papers are available.

Vapid Labs - Lists vulnerabilities, white papers and utilities.

Rscan - Tool that lets a system administrator run complex or simple scanner scripts on any number of machines in a heterogeneous environment. Unmaintained since 1995.

Phrack E-Zine - The well known Phrack Magazine, that's still maintained

John the Ripper: Password Cracker - John the Ripper is a password cracker, currently available for UNIX, DOS, Win32. Its primary purpose is to detect weak UNIX passwords.