Intrusion Detection Systems
Cisco IDS - A network layer intrusion detection system based off of tcpdump with signature support.
Snort - A network layer intrusion detection system based off of libpcap with signature support and preprocessor support.
OSSEC - A host-based intrusion detection system that utilizies log analysis combined with integrity checksums and rootkit detection engines.
Samhain - A file integrity checking application similar to OSSEC
Nepenthes - A malware connection utility similar to HoneyD
HoneyD - A tool for collecting malware and tricking attackers into thinking they've performed a successful attack.
IDS is part of a series on countermeasures.