Undertank Heat Mats are low wattage heat mats that can be used under the tank as well as on the side of the tank. The ones shown here from Zoo Med
have an adhesive backing and are applied right to the glass of the tank. If used under the tank, the tank must be raised on the rubber feet (provided) or some other arrangement to allow air to circulate under the tank and prevent over heating under the tank. These do have the disadvantage that once they are adhered to the tank, they cannot be removed and reused safely, so it makes cleaning and rearranging more difficult.
These mats come in a variety of sizes and wattages. There are guidelines on the packaging for the size of tank the mats are appropriate for heating. The heat achieved in your tank will depend on the ambient temperature and your set up (type and depth of substrate, for example). For tropical or basking species that need higher temperatures, these function best as a secondary (perhaps 24 hour) heat source, used in conjunction with an overhead heat source for basking. They can also be used with a thermostat or rheostat.
Photo courtesy Zoo Med Laboratories, Inc.