When I was broke / going through my hippie phase, this was how I washed my hair. If you use castile soap to wash your hair, you mix about a 1:5 ratio of vinegar to water and slowly pour it over your scalp to get the soap residue off. A minute or so of extra rinsing with just water, and you couldn't smell the vinegar at all.

It was actually kinda nice, and it was way cheaper than silicone/SLES shampoos and conditioners. These days, I just use whatever I have in the shower because washing hair with castile soap is more time-consuming.