My side project is a programming statistics service: https://codestats.net/

It's mainly for fun and I want to keep it free, but of course I wouldn't mind if there was some money coming in to pay for the costs and motivate to work on it more. Currently I'm selling stickers and in the future I will implement some kind of paid accounts which will have some minor features that free accounts don't have – the dilemma is to keep it balanced so that free users don't feel second class.