StopHack is a simple to use and easy to install intrusion prevention system. It is fully adaptable and easily customized to your environment. It is built on top of proven bandwidth arbitration technology so the traffic passing through it won't be slowed down. Every packet is analyzed with regular expression-based behavior anomaly detection, and hackers are blocked immediately. It prevents reflected cross-site scripting, SQL injection, directory traversal, reflected URL redirects, login brute forcing, remote shell execution, and more.
pyHIDS is a host-based intrusion detection system. It uses an RSA signature to check the integrity of its database. Alerts are written in the logs of the system, and can be sent via email to a list of users or on IRC channels (through the irker IRC client). You can define rules to specify files to be checked periodically.