There are countless tools and software stacks out there to monitor your servers out, though some are not the easiest to setup. I went with Nagios since it's straightforward to configure and easy to setup. For reference, I used Nagios 4.0.8 on Debian 7 using Debian 7 slaves. The Nagios master node already had lighttpd and PHP set up so I'll gloss over that.
Master node
Start by downloading Nagios and Nagios plugins:
wget http://prdownloads.sourceforge.net/sourceforge/nagios/nagios-4.0.8.tar.gz
wget http://nagios-plugins.org/download/nagios-plugins-2.0.3.tar.gz
Extract, compile and install Nagios: