Munin is a system and network monitoring tool that can help to analyze resource trends and "what just happened to kill our performance?" problems. It is designed to be very plug and play. A default installation provides a lot of graphs with almost no work.
I use Munin monitoring for many years now and won't miss the raven's great support for my daily work as system administrator. In reward I contribute documentation, code and support to the project since 2007. I wrote a book for sysadmins about how to setup, use and enhance the Munin Software in 2008, put a lot of work into the later within the project developed MuninGuide, created and improved some Munin Plugins and contributed the first version of the Munin Plugin Gallery which was improved since then in a wonderful way by Lars Kruse.