To quote from the original "I've heard many of you love to create side projects for fun, but I also heard that few of them actually took off and grow into a profitable business. So feel free to list if you have any. Statistics will be welcomed as well!"
[1] https://news.ycombinator.com/item?id=1772224
I wouldn't say that this project of mine is "big" in the canonical sense, and it is only meagerly "profitable" by a loose interpretation of the word; but this has been a huge side project from my personal point of view. Essentially, I started this when I was teaching high school full-time and needed a better website for my calculus class. The "big" impact of this project is that it opened a door to a new career for me: I'm now a full-time software engineer and thus measure its profits on the order of my current salary, which is hugely significant to me.
Started life as a side project of our consulting business, is now all that we do. Not big as in making us millionaires big, but it's a profitable business. I wrote about a lot of the story of Perch and the stuff we learned in my book (another side project!) http://rachelandrew.co.uk/books/the-profitable-side-project
A Heroku add-on I made at assist with SSL installation. Really deliberately chose to build in someone else's ecosystem where I wouldn't have so much to do myself.