I have been using Cacti, a RRDTool-based graphing solution, to monitor my servers/VPS over the last year or two (and before that, with MRTG). My needs are simple — I just need graphing of several server metrics and Cacti fits the bill perfectly when the data is well supplied. Cacti is installed on one of my servers, and it will pull stats from my other servers all around the globe at 5 minutes interval.

Data collection from the actual servers is another story. Usually I need

• Number of bytes transmitted and received on all network interfaces except local.
• Current load average over 1 minute, 5 minutes and 15 minutes.
• Current memory usage.