Config Security & Firewall (CSF)
Absolutely the best software I’ve used to date, automatically detects a plethora of patterns and automatically adds the IP to the iptables block list. Has lots of extra features for detecting malicious file activity and SU logins as well as some basic checks to make sure your configurations are secure.
Extremely effective and useful module for automatically blocking IP’s that request the same file very rapidly.
SNORT Intrusion Detection
Very effective and useful tool to monitor everything thats going on in your system and track down potential attempted intrusions.