I use MiniMonitor. It's not the most precise (it was out by 20MB compared to my last bill,) but that could be the difference in when they calculate my usage, and the programmed date to reset the counter.
I consider myself a medium core user and only clocked 80MB last month. It will be way up this month, but I have 1GB so I'm not worried about it. I can't imagine what streaming radio would do to that total... OK, I can, but that's beside the point.
go to www dot ehnert dot net slash minimoni slash (sorry, I can't post links yet.)