https://cronitor.io started as a weekend project. I had an acute need for the product which enforced getting a working version quickly. After using the result of that first weekend hackathon in production for a couple of weeks my co-founder and I decided to put in another couple weekend afternoons and evenings to make it a saas product. We've been working on it part-time ever since.
I wrote more about it in an Indie Hackers interview a few months ago - https://www.indiehackers.com/businesses/cronitor