Sounds super simple, but was awesome at the time. In the 90s when all of my music was either on CD or ripped to MP3, I built an FM transmitter to broadcast my computer audio (sonique or winamp) to any radio or receiver in the house. It was a perfect solution that didn't take long to implement. I didn't know anyone else who did this.

About 5 years ago, I had a car stolen and lamented what affordable tracking mechanisms I could use. I cobbled together an extra cell phone and a data only SIM. I kept the phone running in the back of my van plugged into an auxiliary cigarette lighter port. It uploaded data to google spreadsheets every 15 minutes. I had to root it to have it automatically boot when connected to power. In the end, it was flawlessly reporting its location every 15 minutes. While I was testing this, my car was stolen. The google spreadsheet pointed me to the GPS location where it was. A phone call to the police and a 40 minute wait for them to arrive got my car back only hours after stolen. Dude was sleeping with a big knife next to him, so I'm glad I let the professionals speak with him.