It isn't, though part of the game should be available at the Underdogs. However, I believe your friends are free to offer you a copy of the game, due to a very unusual EULA; it is your best bet to get the game actually, since it has been discontinued by the publisher. There is also the Babylon Project which allows you to play Freespace, but only the Babylon conversion, and not anything else.
And if you meant "free" as in "freedom"... Well, that shall be your goal! Secure space! Crush your foes! You are the deadliest weapon in the universe, able to take down juggernauts by the hundred on your own! Literally, if you are patient enough (two hours and counting to take down the biggest vessel in the game). Fighters were worth their grain of salt in Freespace.
