There are two types of free satellite channels. The vast majority are 'free to air', which means that they're unencrypted and can be received using any suitable satellite box. A very small number are 'free to view', which means that they don't require a subscription but they're encrypted (so you need a valid viewing card).
According to Wikipedia (which seems to be up to date) the following channels are 'free to view' (so you'll need a 'Freesat from Sky' card in order to view them):
Sky 3
Five USA
Five USA+1
Fiver
(Channel 4 HD &Channel 5 HD are also 'free to view' but, unless you've got an HD box, they're not relevant anyway).
http://en.wikipedia.o...st_of_channels_on_Sky
So, if you can live without those few channels, you don't need a 'Freesat from Sky' card. If you do want one, phone 08448 244 400 and Sky will send you one for £25.
Chris