Android: 3G Watchdog
I try to avoid 3G usage as much as possible, especially since I am not on a data-bundle. There are the few cases, where I need to hop onto the internet via 3G, but would still like to monitor the bandwidth usage.
A nifty little Android tool called 3G Watchdog is available, which installs as a notification-bar icon and monitors your usage:
You are able to customise tresholds and alerts for your data-plan and monitor your mobile internet traffic usage (3G / Edge / GPRS) and get detailed status of your daily, weekly and monthly usage. The notification icon will only show when connected via mobile internet. In conjunction with APNDroid, probably the two best (small and free) tools for any Android phone.