How to monitor my data limit on a mobile broadband connection?

There is a package for this called modem-manager-gui:

modem-manager-gui screenshot

Canonical's repositories currently contain recent versions for Ubuntu Trusty and newer. For Precise (and earlier) use this PPA:

sudo add-apt-repository ppa:linuxonly/modem+manager+gui 
sudo apt-get update 
sudo apt-get install modem-manager-gui

See the official site.

You can use NETWORK TRAFFIC MONITOR.
There is Datafox, an addon for Firefox to monitor traffic on firefox.

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