From DD-WRT Wiki
Adds two pages of information showing real-time bandwidth use. When used as a (part of a) startup script one or more small packages are downloaded into WRT's /tmp.
This is a great bandwidth logging app that runs on the WRT54G. The latest version will backup to ftp or jffs and will restore automatically after a reboot etc. Very good for people with limited bandwidth per billing period.
You should already have: -a Linksys WRT54G product -DD-WRT installed as the firmware
short version (cut'n paste from http://www.hetos.de):
tar -xzf wrtbwlog.tgz
open browser to:
See the links below for more details.