Here is the controller I use on all eight of my tanks.
Aquarium Heating & Temperature Control: JBJ True Temp Digital Heater Controller
There are other similar ones on the market but I like this one because of:
1) The large LEDs showing the current tanks temperature at a glance
2) The long removable cord on the temperature sensor placed in the tank
3) The orange activity LED
that shows when the controller is activated
4) The ability to handle up to 1000w of power (3 - 300 watt heaters if needed)
5) That multiple heaters all turn on/off at once (evenly heating the tank)
6) That you no longer need to worry about cheap internal heater locking "ON"
Basically you turn up any attached heater to a temperature a few degrees above where you set the controller. Then you set the controller where you want power activated. When the temperature drops below that point power is feed in to the heaters (which all fire immediately) and will run until the controller cuts off power again. This keeps the heater internal thermostats from turning on/off which can stick and fry your fish.
I would much rather trust the electronics in a better controller than the cheap stuff they throw inside a heater. On my larger tanks I run multiple canisters each with a 300w ETH heater all connected to a common controller.
The only down side is that the controllers work on a 1 degree swing each way. If you set the controller to 75 degrees for instance it will fire when the tank temperature drops to 74 and shut off when it reaches 76. That might not be fine enough for some species...............