Theres not many free foods, the free stuff is mostly veggies and things like spices, drinks etc.
Egg whites are free... exciting I know lol But I often make up scrambled egg with them (it looks so bland but tastes good), or you could make an omelette with veggies.
The only other 'foodstuff' which is free (as far as I know) is jelly oh and rhubarb!