Yep...
MRTG could be a start. At least i know the ISP Singnet in
Singapore is using it. I am also using it now for my attachment
place.
I was asked to link it with html and perl script to make it
easier to configure thought the internal network without having to utilize
the VI editor which any windows users might not know how to use.
Running the crontab will allow you to run the mrtg programme
when you specify it to run, other than that, you can also
use the daemon in mrtg together with interval rate in MRTG
to do the same effect as a crontab will do. OR, a script to edit the crontab will do just fine.