Well, i 'm biased because I like 3g Watchdog, I'm also not quite smart when it comes to phone workings and the inards so to speak.
3G lets me set up weekly/monthly data use. set a start date and weekly/monthly use. The Appdoes the rest and tells me aas the month goes by how many days left, % of my data allocation used, and if I continue at the rate I'm going how much will be left at the end of the month - or when I will use it all.
It can run out of site and open from the drop down task bar, or icon on a home page - I've only used HTC desire and Sensation.
Also options to monitor data roaming as well if you travel.
That's all I know, but it does a lot more, you can have graphs, or whatever. I stick to simple words!!