Recently I switched to carrier to Oister to get cheaper prices, but still get to use 3's network.
However, with cheaper prices often less functionality follows. In my case I've got a limit of 5 GB data usage per month which should be fine. But actually knowing how much data I use is still better but Oister doesn't provide the total usage amount, just how much you've used on each connection.
So I wrote a small Chrome Extension to counter this. Go to "Forbrug" and expand the "XXX gange Data" row. It will then show a green bar for each usage compared to your total use, and in the top it will display your total use, in megabytes.