I have a samsung tv with built in freesat and a sky box, i've had the sky account (though it;s a newish box) almost 10 years, but after the switch to digital sky wanted me to buy a new card for about 20 quid to recieve freesat, as i did not want to pay a sky subscription for loads of their channels i switched the aerial from the sky box to the wall aerial and undid the scart rom the tv to the sly box and now i have freeview channels without any subscription or extra box.
Just to clarify the above answers - there are 2 separate services using the Freesat name. There is the free to air "Freesat" system similar to Freeview, which is the one built into TVs. There is also the "Freesat From Sky" system, which you need a Sky box for and they charge £25 or so for a viewing card.
Some of the channels are different on both platforms, but probably nothing significant!