My 2nd year at SUU I was living in a house with 7 other people. We decided to split the cost of a hot tub. Since we were only going to be at that house for a year we wanted something cheap we could get rid of when we moved out. I found one on KSL for $70 with a broken controller and heater. I was able to fix the heater by opening it up and removing the mineral build up. I then stripped the faulty control circuitry out of the box and replaced it with 3 solid state relays controlled by an Arduino connected to some temperature sensors I got from Texas Instruments. I also made a new control panel for it out of stainless steel with a 16x2 character screen and waterproof buttons.
Total Time: ~10Hrs