remove the air silencer (the rubber cone that comes out of the intake box and goes into the fenderwall). It won't gain you any real performance per se, but it will make the car sound a lot better. With my K&N filter, it really gave my 2000 GT a 4 barrel carb type sound.
Other than that, I don't think there are really any free mods you can do to gain power...if there is, I want that list too.
"When I know more, I'll be forthcoming. Or I won't be forthcoming, and I'll be honestly deceptive." - Les Miles