Download Monitor is an application developed for the Ubuntu Apps Showdown, useful for computers with a limited Internet plan.
The network monitor application displays monthly, daily and hourly amount of data transferred over the network and it can warn you using desktop notifications when you're about to exceed the usage quota, which you can set from its preferences.
Download Monitor integrates nicely with Unity by using dynamic quicklists so you can see the network usage with a click and also, there's an Unity Launcher progress bar which displays the used / remaining usage quota:
Once the Ubuntu Apps Showdown is over, Download Monitor will be available in the Ubuntu Software Center but if you want to install it already, you can use the following commands:
For other Linux distributions, you can get Download Monitor via Launchpad.
sudo add-apt-repository ppa:duncanjdavis/download-monitor-submit sudo apt-get update sudo apt-get install download-monitor
Also see: Network Traffic Monitor (NTM) For Computers With Limited Internet Plans