http://www.chaosreigns.com/code/linodebw/

This script monitors your bandwidth usage 4 ways:

• local: total monthly bandwidth usage estimate based on data from /proc/net/dev over the last three hours
• month: total monthly bandwidth usage estimate based on linode xml over the current month
• period: total monthly bandwidth usage estimate based on linode xml over the last three hours
• used: actual total bandwidth used this month, from linode xml

If any of these goes over a threshold (percentage of your quota) specified by you, it emails you. All calculated rates are logged as gigabytes per month.

The url lists three files:
linodebw.pl - perl script, you need to edit three variables at the top.
linodebw.png - example output from my linode
linodebw.gnuplot - config file to graph the data with gnuplot, you'll need to edit the output and log paths.

I would appreciate feedback.

Original Linode XML announcement: http://www.linode.com/forums/viewtopic.php?t=776
Update: http://www.linode.com/forums/viewtopic.php?t=1558

Google searching turned up a previously created script by the same name, but since it appears to have vanished I re-used the name.

Alternatively, look up vnstat. It binds to a specified eth interface and monitors all I/O traffic. Statistics can be displayed with a PHP script that someone wrote. See, for example, http://thedimi.net/a/vnstat

I second vnstat!