To answer your question properly, you need to understand the difference between 'Free to Air' and 'Free to View'. Similarly you need to know the difference between 'FreeView' and 'FreeSat':
A Sky box, without a valid card in it, will still pick up 'Free to Air' channels. This includes the BBC and ITV channels and all radio channels. However, this is not the same as receiving 'Free to View' channels. To get these stations, you need a valid card in your box. As Alan states, the card will cost a one-off fee of �20, from Sky. This 'Free to View' service is branded by Sky as 'FreeSat'. This is similar to Freeview but the selection of channels is different. For example, you won't get UKTV History, E4 or ABC1. You will, however, still get far more channels than with Freeview (which is only available from terrestrial transmitters).
Details of FreeSat (including channel listings) are here:
http://www.freesatfromsky.co.uk/
The information is mainly aimed at people who will be purchasing a new satellite system (for a one-off cost of �150) to receive FreeSat but the information about the �20 charge for a card to be used with existing equipment is hidden in the FAQs.
Chris