RRDtool is the roundrobin statistics generation tool from Tobias Oetiker. I like it very much because you can graph almost everything with it. But keep in mind, you can spend very very much time on it ... .

Displays the internet traffic in bits per second, ...

... shows the memory usage, ...

... and finally the clients in a network, which are online.

• RRDtool homepage

  The official RRDtool Homepage

  
  
• #rrdtool @ IRCnet page

  Here you can find the website of the rrdtool channel in the IRCnet

  
  
• Grading with RRDtool Tutorial

  Grading with RRDtool - an easy HOWTO by Alexander Krogloth

  
  
• RRDTool on Debian GNU/Linux Tutorial

  RRDTool on Debian GNU/Linux Tutorial - HOWTO by Alexander Krogloth

  
  
• collectd

  Collectd is a small daemon which collects system information every 10 seconds and writes the results in an RRD-file