Openbsd-bwusage is a project mainly written in Python, it's free.
A simple means of monitoring bandwidth usage on a PF-using OpenBSD system.
To automatically update bandwidth usage on a daily basis and then send the latest stats off via e-mail, insert the following line into your crontab:
58 23 * /root/.apps/openbsd-bwusage/update-bandwidth-stats-and-email