All natural (mostly) wood stains! A handful of pennies in vinegar will produce a beautiful pale Caribbean blue stain. A wad of steel wool will give you a rich reddish hue. A combination of tea and a metal object in vinegar will produce a black stain.
Transferring pictures to tiles by using Nail Polish Remover. This is freaking ingenious! And all you have to do is put the picture on the tile and rub the nail polish remover on top of it and it will bleed through!