Fail2ban is powerful opensource and most popular intrusion detection and prevention system that can block attacks being launched on your linux based system/VPS/server/cloud etc. e.g: if you have SSH service running and someone tries to brute force this, fail2ban can detect it and even can block such users (using the ip address).
OS: Almost available on all flavor of Linux.
Installing and Configuring Fail2ban
This gig offers basic installation and configuring of fail2ban.
(1 service only)