White cloud minnows? Red rosy minnows? Dojo loaches? Hillstream loaches?
Really though unless you love some of the above fish and your tank will stay within the right temperature range (and remain at stable temperatures - you don't want it frequently fluctuating) than it would just be easier to get a heater. They aren't a lot of money and won't add much to the electric bill provided you don't have loads of aquariums and this way you'll have more choices since most fish will need some heat during the cold months. Unless your house is really well insulated maybe(?).