Having a system for collecting performance statistics can be extremely useful. In the past, I've used Munin for this, and it has been invaluable in analyzing bottlenecks and various other issues. I was recently made aware of collectd, which seems very similar to Munin.
What monitoring applications are the major differences betweenavailable and should be considered (other than Munin and Collectd), and how do you choose which one to use? Are there other monitoring applications that should be considered?