I made a tool that sends you an alert if your SSL/TLS certificate is due to expire.
This came about because over the last year I have had a few certs expire. Also I had a discussion about agile and minimum viable product.
That got me thinking about what could be created in a day.
The results of this became CertAlert.net.
Written in Golang in about 10 hours, all it does is check your TLS cert once a day and sends an email if your cert is due to expire in next X days.
There are many such tools but most want your infomation for marketing, CertAlert is driven by donations via ko-fi.