Pretty simple Shopify app, sitting at about $2700 after expenses. I started it back in 2013, and it slowly grew but has now leveled off. Pretty sweet considering I only answer a handful of customer requests a month now.
I don't sell anything on Shopify, I just created an App for their app store. It helps people manage their inventory. It's actually stupid simple, but it was a need a lot of my customers were asking for at the time.