Free: Contests & Raffles.
I did the frozen 2 liter bottle thing, they seemed to really like it. You have to have a bunch though, as I recall they like chewing holes in them once they thaw!!!
thinking about coiling up some 1/4 inch copper tubing and gluing it to the bottom of a 12"x12" porcelain tile, then gluing a pine backer board to insulate and protect the copper tubing. Like a sandwich with the copper coil in the center. Then I'll run water from the copper coil to a mister and run cold ground water through it. The mister would be setup so it wouldn't get the rabbits wet, just cool the hutch roof and general area.