Reichotron 3000
I came across this clap detection library some years ago, and it made me want to create Reichotron 3000: a piece of software that detects the rhythm ♪♪♪𝄾 ♪♪𝄾 ♪♪𝄾 ♪𝄾 and starts clapping along with you.
I've never found the time or inclination to write this. If you need an idea for the next Stupid Shit No One Needs & Terrible Ideas Hackathon, why not steal this one? Someone's made an iOS app but it doesn't sound quite the same.