The Good Guys

CERT is the standard place to start: http://www.cert.org/ and http://www.uscert.gov/. Browse their past advisories, and get on their advisory mailing list.
Also check out CERT's:
- Intruder Detection Checklist ftp://ftp.cert.org/pub/tech_tips/intruder_detection_checklist
- Steps for Recovering from a UNIX Root Compromise http://www.cert.org/tech_tips/root_compromise.html
- UNIX Configuration Guidelines ftp://ftp.cert.org/pub/tech_tips/UNIX_configuration_guidelines
- List of Security Tools ftp://ftp.cert.org/pub/tech_tips/security_tools
- What's New and Updated http://www.cert.org/nav/whatsnew.html
FIRST — Forum of Incident Response and Security Teams: http://www.first.org
DOE Computer Incident Advisory Capability: ftp://ciac.llnl.gov/ciac/CIACHome.html/
NAVCIRT — Naval Computer Incident Response Team, and SPAWAR — Naval Information Systems Security Program Office: http://infosec.nosc.mil/infosec.html
USAF CERT http://afcert.csap.af.mil, with FTP archives at ftp://afcert.kelly.af.mil/pub/advisories/
NASIRC — NASA Automated Systems Incident Response Capability: http://nasirc.nasa.gov/NASIRC_home.html
FBI National Computer Crime Squad http://www.fbi.gov/programs/nccs/compcrim.htm

AUSCERT - Australian Computer Emergency Response Team: http://www.auscert.org.au/
Australian Defence Signals Directorate: http://www.dsd.gov.au
German Federal Networks CERT: http://www.cert.dfn.de/
CERT-NL, SURFnet Response Team, Netherlands: http://www.nic.surfnet.nl/surfnet/security/cert-nl.html
SWITCH-CERT - Swiss Academic and Research Network CERT, Switzerland: http://www.first.org/switch.html
Italian CERT: http://www.sec.dsi.unimi.it/
Other Euro-CERT's: http://www.cert.dfn.du/eng/csir/europe/certs.html
Hong Kong Police Commercial Crimes Bureau: http://www.info.gov.hk/police/
Royal Canadian Mounted Police computer crime efforts: http://www.rcmp-grc.gc.ca/html/cpu-cri.htm

US DoD folks, check out DISA: http://www.disa.mil/.
NCIX — National Counter-Intelligence Executive has newsletters on information security and more general but related topics: http://www.ncix.gov/
NSA is the information security agency in the U.S., and includes the National Computer Security Center, source of the "Rainbow Series": http://www.nsa.gov/:
As for the documents themselves, check:
National Security Telecommunications and Information Systems Security Committee: http://www.nstissc.gov/
NIST Computer Security Resource Clearinghouse: http://www.csrc.nist.gov/
DOD Security Institute: http://www.dtic.mil/dodsi/
US Army Information System Security Office: http://www.army.mil/disc4/ISEC/ISEC.HTM
International Computer Security Association: http://www.icsa.net/
National Security Institute http://nsi.org/
American Society for Industrial Security. http://www.asisonline.org/ and http://www.securitymanagement.com/
UK folks should check the UK ITSEC scheme: http://www.itsec.gov.uk/
Aussies can check their national Evaluated Products List at: http://www.dsd.gov.au/epl/

Hey, so I'm biased, but Purdue'CERIAS project has a lot of great information, tools, pointers to other resources, etc.: http://www.cerias.purdue.edu/
The University of Texas has a nice collection they call the "National INFOSEC Information Mall": http://niim.bus.utexas.edu/
Other great tool collections are: