Ack, still a bit too recent and bit too raw but here goes.
Last side project which ended up becoming a bit of a rollercoaster was Brring Conference, a conference calling app where you could schedule conference calls ahead of time and it'd send out calendar invites and then automatically dial everyone in when the call time arrived.
https://www.brring.com
At the time of inception I noticed that the UK had terrible conference calling providers. They'd bill ~£50 for a one hour international conference call with 10 people. I did my numbers and realized I could provide the same call for 1/10th the price using outbound dialing and least cost routing via Twilio/Plivo etc.
I thought I was off to the races but I made the cardinal mistake of making every startup mistake in the book.
It took a few months to build a basic prototype and then using lots of my own money (and some begged, borrowed and raised) it slowly grew to become quite the monster in my life.
I wasn't a complete idiot though, I ran the idea by whom I figured my target market would be, wrote up a business plan, projected costing/break even and looked up how much my competitors were making via their company filings - the market (at least in my eyes) was there.
By the time I launched it was too little too late - our big competitors in the UK (and US for that matter) had completely revamped their products in response to Microsoft Teams. As an example of how competitive things became Google Ad spend for the term 'conference calling' literally went up 10x when I launched in response to competition between Slack, Microsoft and our own local competitors.
We obtained 100+ users and two dozen paid within the first month but the pace didn't really pickup much beyond there so after several months of adding features and running ad campaigns instead of parking it I decided to pivot the whole thing and turn it into a lead generation widget - like Intercom or Drift but supporting phone and WebRTC calling via the browser to a sales agent.
Things did better this time, I signed up paid customers ahead of time to figure out what features to launch with and ensure there was a market there once our product arrived - I was pinging maybe 30+ business owners a day asking if they'd be interested and I managed to get hoteliers, estate agents, cruise ship companies and travel agents interested in using my product.
We built Brring Live Dialer in a few months and things were growing better. Then COVID hit and our customers just evaporated. I decided I'd had enough of this side project which had become my life and slowly wound things down and started interviewing.
I have to say the experience of the above was instrumental in leveling up my resume. I came from a background of XMPP and telephony - where our software was deployed on-prem. I'd allowed myself to become deskilled so building an app from scratch using the latest web technologies of the time meant I had zero issues interviewing and landing a job.
Still smarts though, making so many ruddy mistakes!