Any Sky box should be able to receive 'free to air' (FTA) channels. Such channels aren't encrypted, so you don't require a valid viewing card.
Nearly all of the free satellite channels are 'FTA' but a few are 'FTV'. (Free to view). Such channels are encrypted, so you need a valid viewing card to watch them. Sky used to offer a suitable card for a one-off payment of £20 but I believe that this has now been discontinued. (When Sky initially sold the cards, lots of channels were 'FTV' but most have switched to 'FTA', making the cards largely redundant).
At the present time, only Sky3, Fiver and Five US require a viewing card. If you can live without those channels, you can use your Sky box to receive all the stations listed here:
http://sam-radford.me.uk/ftv.htm
Freesat is slightly different. You can use your existing satellite dish but you would require a new digibox. You can choose between standard definition (SD) or High Definition (HD) models, depending upon whether you'll be using an HD-ready TV set. Details are here:
http://www.freesat.co.uk/
Chris