I used nagios (still do) but I found it frustrating and complecated to set up.

I started writing little one-off perl scripts that send an email for the alert.

However, managing and monitoring the email folders is tedious and taking up too much time.

xymon/hobbit (http://sourceforge.net/projects/hobbitmon)

The name change was due to trademark issues (http://www.hswn.dk/hobbiton/2008/11/msg00123.html) ...not sure Gandalf would aprrove, but it keeps lawyers happy.

Nagios and Cacti.

Used to use Big Brother.

munin and pingdom.com