I use Network Meter and I like it.
I installed it on all my pc's so it will track usage that I can (add up) each month to get a "rough" idea of how much data has been sent/received.
This, of course, does not include any traffic from my two ps3's, my AVR, or the smartphones.
I like the fact that on Network Meter you can reset the counter manually or set a date for it to reset.
It would be nice (I have seen routers like the ASUS) that actually have traffic measurement/reporting.
That is cool.
Maybe someday DLink will do that - but so far their UI has not changed very much - and I would suspect that short of just "fixing bugs", they are not interested in creating a better, more useful UI - but of course, time will tell.