If you're sufficiently into DIY and electronics, a programmable temperature probe could automate pretty much any sort of heating unit, be it submersible, pad, or heck, even an electric blanket wrapped around the bowl.
I think most people would find that a good adjustable submersible heater is by far the simplest and most effective option, but anything that produces hear can be made to work with enough know-how.
Under-tank heating mats are usually used for terrariums (reptiles, mostly), and many have dire warnings about using them anywhere near water because they're not very well insulated and could lead to electrical shock. But they could work, if plugged into a programmable thermostat with a temperature probe. Seems like a very inefficient and expensive way of heating a tank compared to the more traditional heaters, however. I can understand why if you a specific look you're trying to maintain, and can't have a normal heater in the tank (and it's accompanying wires), but it's never going to be an optimal solution.