There is a new Intrusion Detection System for FreeBSD. The thesis is available. From the email to the IDS list:
The IDS system is designed as a kernel module for FreeBSD 5.2. It is inspired by the SpamAssassin program, which detects spam by applying a set of tests to every email message and counting a sum of point score generated by each test. My IDS system applies a set of tests to every running process in the OS and counts its score generated by the tests. Therefore, the purpose of the IDS is not to monitor the network traffic, but rather to monitor the process activity.