Baking soda. Mix a teaspoon or so with about a cup of warm water. You can pour it over your hair or apply it directly to your roots with a squeeze bottle with a pointy applicator tip.
I don't think this will get off 'cone build up, but it will help with gunky buildup from other products. I read in another post that baking soda is alkaline and will leave the cuticle of your hair open, so it might feel a little funny afterwards, but not like a sulfate would leave it.
I didn't notice anything, except wonderful, build-up-free roots!