Intrusion Detection Systems

Talisker Security Wizardry - Large index of computer security resources, including a glossary of Intrusion Detection and Prevention systems.
http://www.securitywizardry.com/

DShield - Distributed Intrusion Detection System. Accepts firewall log excerpts from around the world and you can browse through activity reported to the system. Features a list of the "most wanted" abusive IP addresses.
http://www.dshield.org/

COAST Intrusion Detection Hostlist - Clearinghouse listing of many of the Internet resources associated with intrusion detection and prevention systems. Includes inventories of commercial products, research projects, and other IDS-centric material.
http://www.cerias.purdue.edu/about/history/coast_resources/intrusion_detection/

Security Resources for Windows Network Administrators - Provides books, articles, software and other information on penetration testing and other security issues. Geared toward Windows NT/2000/XP Network Administrators.
http://is-it-true.org/pt/

Linux Security IDS - A source of IDS-related information, including security best practices, audit procedures, penetration testing, and community information.
http://www.linux-sec.net/IDS/

myNetWatchman.com - Intrusion Reporting and Response. Users forward firewall logs that are aggregated and analysed to identify incidents that are reported to the responsible party.
http://www.mynetwatchman.com/

LinuxSecurity IDS Resources - Collection of IDS-related resources made available by the linuxsecurity.com community.
http://www.linuxsecurity.com/component/option,com_weblinks/catid,117/Itemid,134/

Michael Sobirey's IDS Page - An excellent index of most currently available IDS and IPS products, including vendor contact information.
http://www.cse.sc.edu/research/isl/mirrorSobireys.shtml

Internet Storm Center - Operated by the SANS Institute, the ISC provides a user-supported infrastructure for intrusion detection systems to share information about ongoing attacks that span countries, networks, and administrative boundaries.
http://isc.incidents.org