If your are after something to monitor your 3g data allowance, 3G Watchdog is pretty good. You can set your limits and monthly renewell date, and it'll warn you if you get near your limit, and turn off 3g if you reach it.
On the other hand, Traffic Monitor logs all data, 3g and wifi, and very usefully logs which app is using what. Very useful if you suspect one app of chewing through more data than it should. You can also test network speeds with it.
Sent from my HTC Desire S using xda premium