Been posted before but quite a few months back now so as I just ordered another for myself I thought I'd post as may be useful for those who'v enot seen this offer before.
Just fill the form in and you'll get a free Datasim in the post. even P&P is free
then register the Simcard on the three website and you'll get a fresh 200Mb allowance credited to to the sim every 30 days. you can top up as you wish If you need more data but even without a topup you will still get the 200Mb free every month.
I've been using one in my Android car stereo for Google Maps use for almost a year now.
w w w.three.co.uk/Free_SIM_MBB/Order