A newb question, as I just received my wife's HTC Desire: Where can I check how much bandwidth it uses (or used during certain time period)? Her contract only includes 500MB a month, so she needs to be able to monitor this.
Mentioned 3G Watchdog probably is the best choice. Alternatively, you might want to take a look at...
...which also cover your needs. Might be you like one of them more :)
Talking about more, I obviously missed...
Have a look at 3G Watchdog. You can add a homescreen widget, and even disable 3G once you get to a predefined limit.
Really surprised this wasn't posted already, but check out My Data Manager.
I am very happy with it! It has a notification view of data used on both network and wifi that only shows when you expand the bar (so it's not annoying).
It has support for how much data your plan is for, when your bill comes, and has great graphs of each application's usage, and even time running!
Onavo has generally been well received, both on iOS and Android. It has widgets and some fairly nicely drawn bar graphs to make the data easier to digest.
You can also pair it with their other offering, Onavo Extend, which will compress your requests and route them through a proxy server in order to save bandwidth (similar concept as Opera Mini). It's worth noting, though, that if you're truly concerned about privacy issues then you may want to steer clear of Extend, as all of your data will pass through Onavo's servers (though they say they don't monitor/collect it, of course). They claim up to 5x compression rates.
Reto Meier did a talk during last Google I/O where he spoke about it:
Watch around 25 minutes.
- Logcat logging
- ARO tool from AT&T
- Network Statistics in DDMS