Want to make sure your server is up? Dying to know why the load is 5+ on a 2-core VPS? Using monitoring software.
Yes, it generates overhead. Yes, it is a pain to install and configure sometimes. However, when you want to know why something is breaking or how something is happening, it can be your best friend. Nagios and Cacti are two that I swear by daily, and love using for every bit of usage I can drain out of it.