I launched MageTools (https://mage.tools), an alternative to Chrome's DevTools inspector, about a week ago. I don't have that big of an audience, but I posted it on my Twitter, in wip.chat, and on my personal website and hoped for the best.
After a few days, I'd gotten 36 free signups. I was a little bummed, so that Saturday, I decided to drown my sorrows by playing a video game. A few hours into it, I saw my phone lighting up from across the room. It had a bunch of email notifications:
I thought someone was trying to hack me, so I stayed up until 3 AM to monitor the traffic. I wrote a script to check if the license keys were actually being validated, and to my surprise, they were! These were real users. Apparently, MageTools had been posted on a foreign social media site and seemed to be popular.
It has since been a couple days. I've gotten random bursts of signups ever since, and just passed 200 users. Of course, this doesn't make the business valid yet – I've been offering a 100% off coupon for early adopters – but at least it's a start!