I build products to help improve my main e-commerce businesses so I think this counts as a side-hustle. The latest (earliest stage one is):

https://www.zigpoll.com

"Bite sized" polling software as a service that I use for post purchase surveys, contact us forms, and email campaigns when I want feedback on what to do next. Most customers are currently through our Shopify App but have a few SaaS businesses that integrated it independently.