I used parse-server and services back when it was a web service at an internship, just loved the way it did things it did. Backend as a service was new to me as a mobile application developer. 5 years down the lane. My first go-to backend is Parse. I know firebase does XYZ things better. But I love the simplicity and openness of parse.
Community picked up parse as a self hosted open source service and its still going strong.

Just love the possibility of starting a mobile project and not having to worry about setting up a whole web service to cater to it.