ScanKeeper is an offline loyalty card app for iOS and Android. Solo build, no funding, no team.
One post last week did over 1,500 downloads in 24 hours. The number I keep staring at isn't that one though, it's the 17.7% conversion on the store page. Traffic you can always find somewhere. People installing after they've actually read the page is the product doing its own work.
It also pushed the app to 8th on India's top-grossing shopping chart, which is shelf space I could never have bought.
Today it's on Product Hunt: https://www.producthunt.com/products/scankeeper
The part I've never handled well is the day after a spike. If you've been through one, I'd like to hear what you did with it.