The Multi Router Traffic Grapher (MRTG) is a tool to monitor the traffic load on network-links. MRTG generates HTML pages containing graphical images which provide a LIVE visual representation of this traffic

Although it talks about "network traffic", it has been used to monitor lots of other things, like weather, surf, temperature...

MRTG optionally can work with rrdtool or use its own file-based storage.

While it does not directly give averages, it provides a great graphic display, and is very useful for observing data over time.

Yes - it is written in perl. Yes, it is free. Yes, it is good!