Ancient Romans had air conditioning. In the summer, they would seal all their windows and doors, but open a hole in the center of their house down into a tunnel that travelled under their house for 20 feet or so, to the outside, and opened in a shady spot. This underground tunnel contained air cooled by the earth.To draw cold air into the house, the Romans had a black-painted, 3 foot chimney on their roof. When the sun heated the chimney, the hot air inside the chimney (and house) would rise, and pull the earth-cooled air into their home. Since air in underground caves is 65 F year-round, this process cooled their homes considerably.
You may be able to cool your home by blowing the cool air from your basement through your duct work, or by aiming a fan up you basement steps. Alternatively you can freeze water in water bottles, place them in a dish drainer (set atop a towel to absorb condensation), and direct a fan across the ice filled bottles.
If you're feeling a bit more adventurous, check out this air conditioner constructed by a student. This homemade air conditioner uses a fan, an ice water filled cooler and copper tubing. Note: you can pick up the zip ties used in this project at the dollar store to further reduce the cost of this project.