I was working as a contractor, and was looking after a growing number of periodic processes (data imports, backups). I wanted to get actively notified if any job starts to fail (for any reason: bug in code, external system down, out of disk, out of memory, whole VM destroyed by error, ...). I knew the concept of Dead Man's Switch from Wikipedia surfing, and it clicked.
I did not start with the intention of building a microstartup. It started as a fun weekend prototype, then a side project, and over time grew into my main job.
MicroFounder
How did you find your first customers?
David Lynch
Healthchecks initially had no paid plans. The free users came from social media mentions, word of mouth. I posted on Twitter, Reddit, HN. I started a development blog on Medium. Botched a Product Hunt launch. Answered some StackOverflow and Quora questions.