I dare say that only a few sys admins haven’t heard of fail2ban – maybe those starting out or those who have focused on different areas. In my experience, it’s quite rare that really small utilities can affect the way you run your servers to the extent that fail2ban has. It certainly explains its popularity.
fail2ban is a feather-weight set of scripts that can easily integrate with popular firewalls and, amongst many other things, catch any failed logins for services that you’re running and then ban the IP address after a certain number of failed attempts. Admittedly that sounds like quite simple functionality, but when you get down to the innards of the software, it’s a truly powerful tool