Here is a list of some of the tools I like to use in my day-to-day jobs:
Munin – One of my favourite stats collector for *NIX environments.
Cacti – Very useful for grabbing some statistics from those pesky network switches (or just about anything:)).
Nagios (and now, Naemon) – My monitoring tool of choice. Has some flaws but is extremely easy to configure.
Adagios – A brilliant web frontend for Nagios and a configuration editor.