Bartering. Not just for pre-monetary civilizations. As a SAHM, I find my greatest commodity to barter is childcare, but it could be anything. Some things we have bartered: building supplies, yard sale finds, house sitting, pet care, rides, feeding animals for a vacationing farmer, cleaning. Some things we have bartered for: piano lessons, garden produce, handyman type work, cleaning, meat, fresh milk. We are pretty fortunate to live out in the country, where there is an ample supply of things like produce and meat, but bartering could work anywhere.
Bartering has saved us a good deal of money and made us better neighbors.