Intrusion Detection Tools

Assuming you got your system into a reasonable state, you would like to keep it there. Or, at least you would like to know if it is changed!

SIEM or Security Information Event Management is the current buzzword in the field.

File System Change Detection

Tripwire is the best-known solution. They also offer products to check router configurations and modules used by a web server to detect unwanted content change and serve out a "Temporarily unavailable" page in its place.
Tripwire

AIDE (Advanced Intrusion Detection Environment) is a free replacement for Tripwire, included in many Linux distributions.
AIDE

Osiris and Samhain are alternatives to AIDE.
Osiris Samhain

Host Intrusion Detection Systems

OSSEC is a free open-source IDS. There are agents for Linux, BSD, OS X, Windows, and Solaris. It does log analysis, integrity checking, Windows registry monitoring, and rootkit detection. A central data collector and analyzer provides a web dashboard interface. It supports active response, making it a HIPS or Host-based Intrusion Prevention System.
OSSEC

Wazuh is a fork of the OSSEC project.
Wazuh

Falco is a free open-source IDS for containers.
Sysdig | Falco

SNARE - System iNtrusion Analysis & Reporting Environment provides host-based intrusion detection for Linux, Solaris, and Windows including graphical configuration, monitoring, and reporting tools.
SNARE Snare Agents for Linux, Solaris, OS X, Windows

Network Intrusion Detection

Note carefully that most "network intrustion detection" system really detect an attack and not an intrusion. Still possibly useful, just make sure you understand what a tool really does.

Also be somewhat skeptical of the real need for NID. If you are running BSD, do you really care that someone on the other side of the planet is trying to exploit a risk only found on Microsoft SQL Server? Is it appropriate to waste your time being "warned" about this complete non-risk? Aggressive NID can be a denial-of-service attack against yourself!

Security Onion is a free Linux distribution with several network IDS tools plus Wazuh (see above) and other host IDS, testing, and analysis tools.
Security Onion

Snort is a great tool that detects and diagnoses scans, probes, and attempted attacks.

There are many tools to automatically analyze audit trails for suspicious events. Splunk, Hewlett-Packard's ArcSight, and IBM's QRadar are the big names in the field.

Also see Purdue's CERIAS group for current research in this challenging area.