SecurityFocus: Home of the BugTraq archives; security consulting services; links to security/hacking news from around the Web (updated continuously); and fine, original articles by Kevin Poulsen and guest contributors (including El Reg's own, by virtue of a content sharing agreement).
Security News Network (formerly Hacker News Network): Links to security/hacking news (updated daily); original research leading to periodic embarrassments for SW vendors; security consulting services.
PacketStorm: Links to selected security news (updated periodically); free security tools; live exploit code (for research purposes only, please); discussion forums and a vast archive.
CERT: Computer Emergency Response Team at Carnegie Mellon Uni; one of the largest archives of virus reports, exploits, bugs, advisories and network-administrative worst practices on the Web.
Cryptome: Cryptography and surveillance news and documents; government shenanigans involving crypto, surveillance and related technology.
SecurityPortal: News links; advisories; original articles; security services and a nice crypto section.
SecuriTeam: Portal with news; exploits and tools; product reviews; and separate focus areas for UNIX and Win-NT/2K.
Whitehats: Tools; forums; bug reports; news links; and original research articles.
Hackerz Hideout: Big educational site with hacking tutorials covering use of viruses and Trojans, network clients and IP architecture; holes, bugs and their related exploits; a zine archive; and a vast archive of toolz, appz and sploits.
Attrition.org: Big site with news links; advisories; essays; a crypto archive; and an immense archive of defaced Web pages.
IceFortress: Educational site with all the basic toolz and appz; challenges and games; and forums with extremely patient admins willing to enlighten even the most clueless newbie.
Ksoze's Cracking Workshop: Tips, tricks and original source code for advanced crackers.
AntiOffline: Irreverent, satirical zine devoted to hacking news; original articles; and spoofed Web pages.
Fravia+: Very original site devoted to searching/penetrating the Net with creativity.
Deny.de: Educational portal with news; appz; toolz; and forums with patient, helpful admins.
SamSpade: Heaps of useful CGI gateways.
Mixter Security: CGI gateways; and security self-tests.
CyberArmy: Big site with appz, toolz, tutorials; a hacking-related search engine; and challenges and games.