i am trying out 3G Watch Dog for a couple of days now. it's pretty good. it even allows you to put a small widget on the home screen that shows you the current monthly usage. it lets the user set the quota, billing period, and have it automatically reset at the start of the billing period.
i have experienced 2 times now where the AT&T account site was showing about 10mb more than what the 3G Watch Dog was showing. the AT&T account data indicator is delayed, but there shouldn't be any case where AT&T shows more data than the data monitor on the phone.
Try Mobile Counter - application to count network GSM/WIFI transfer (also in blocks e.g round data at least 100kB for every session). Widgets, more settings, application similar to watchdog
More details in android market: https://market.android.com/details?id=com.mobileCounter