There actually was a "free" version of FS2 on DVD, but it was an OEM bundle (FreeSpace 2: Colossus), which means it's not free, it's only free if you got it with some hardware (such as a sound card) as a sort of 'gift'.. a gift that encourages you to buy the whole version, as it's a sort of hybrid. The game has more than the 2 or 3 mission demo you can download for free, however, it cuts off halfway through the game. I remember getting Descent: Destination Saturn in this manner, as an OEM bundle with a Creative Sound Blaster 14.4 modem that my dad got. It cuts off at level 15, whereas the full version of Descent has like 23 levels or somesuch.
Anyways, point being, it's not even legal to download said version of FS2... it's only to be distributed with the hardware that it's part of a bundle with.
Your only truly free version of FS2 is going to be the FS2 demo (with its 2 or 3 missions). If you want that, I can get it for you and upload it somewhere if you can't find it.