Some people seem to be using NetMeter for continuously monitoring their network usage against a limited volume data-plan. Since we are now moving to Europe, where stingy data plans with only a few 100Mbs included per month are the norm, I now also have a sudden interest in measuring my monthly data usage.
Since NetMeter is a trouble-shooting tool, oriented towards very detailed tracking of network usage over the last few minutes to see what is going on right now, it is not really very suitable as long-term bandwidh counter.
The ideal network usage counter should be low overhead and run continuously in the background - e.g. start itself at boot time. In its most basic form, it should show the current monthly total of cellular data usage. It would also be nice to enter the specs of the data-plan to track agains: how much is included per month, what is the rate above that limit and when does the monthly limit reset. For the current month, the ideal application should always show how current data usage compares to the "budget", by constantly pro-rating the monthly limit to the part of the billing-month which has already passed. For people who travel a lot, tracking multiple SIM card accounts or roaming carriers with different rates would be nice to avoid nasty surprised at the end of the month.
Among the free applications on the market which offer bandwith usage tracking, I have tried NetCounter and NetSentry. Both run continuously in the background and poll the linux interface counters in the /proc pseudo-filesystem. Both are capable applications which track usage on both wifi and cellular data interfaces over a monthly period with configurable starting date. At this point neither offers yet an ongoing comparison of actual usage compared to the pro-rated monthly limit or any plotting of historical usage data from previous month for trend-analysis.