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.
OSSEC For PfSense is a module that allows the execution of OSSEC HIDS on the PfSense platform, in order to improve the security of your network. It enables better diagnosis and anticipation of possible attacks on servers. As the OSSEC is based on analysis of logs, its accuracy in detecting potential attackers makes it one of the best tools for intrusion detection. It can also read the logs generated by the Snort package, which can easily be added to PfSense.