User created API consuming dashboards, I drove lots of traffic to it but it was a really niche product and I just don't think there was a path to revenue with such small numbers.
Static pagebuilder before Square and the like had that kind of thing dialed. Still proud of the technical achievement, had some users, but it never took off. I couldn't afford to market it.
A more traditional digital agency I started with some colleagues. It went well, and then it fell apart when one member grossly mismanaged the money and then took some big clients on his way out. I regret not sticking with it, but it was really demoralising that such bad behavior was rewarded and we were left with nothing. Many lessons learned though, I would never allow it to happen again.
Many other small projects that just waned or never got built to completion. I do all of this on my own and with my own money outside of any tech bubble, so it is pretty easy to get derailed. I have been at it for more than 10 years.