The tail Linux command is a very useful command for viewing logs. It is designed to show the end of a file on a screen. It can however be set to automatically set to always display the end of a file. Want to monitor your Debian web server's SSH logins? run this simple command:
tail -f /var/log/auth.log
In particular, I've found it very useful while attempting to troubleshoot problems with an exim4 mail server on a VestaCP installation by watching inbound and outbound email traffic.