Intrusion detection / prevention systems (IDS/IPS)

Intrusion detection / prevention systems analyzes network traffic searching for indications of attacks and malicious intent. IDS only detects possible threats while IPS prevents threats from entering the system
   
Snort open source tool for network intrusion detection and prevention fused for sniffing on a potentially compromised host or network segment
  http://www.snort.org/
   
Basic Analysis and Security Engine Performs analysis of the intrusions Snort detects on your network
  http://base.secureideas.net/
   
OSSEC Open Source Host-based Intrusion Detection System. It performs log analysis, integrity checking, Windows registry monitoring, rootkit detection, real-time alerting and active response
  http://www.ossec.net/
   
SURFids
an early warning system which lets system administrators correlate known and unknown exploits to attacks directed towards their networks
  http://ids.surfnet.nl/wiki/doku.php
   
Panoptis
Network based IDS used primarily for detecting and blocking DoS and DDoS attacks.
  http://sourceforge.net/projects/panoptis/