App marketplaces like Wordpress, Shopify, etc?

I launched EcardWidget.com in January 2019 as a standalone web app. That required a lot of work to keep traffic and new users coming in. I then launched a Wordpress plugin that integrated my product. That brought in a lot more users.

I'd like to do more integrations like this and I am sure there are other platforms that allow 3rd party integration with marketplaces for plugins. Like Shopify and Wix for instance. However I am having trouble finding an exhaustive list of these platforms. Any body have link to a list or integration markets that I'm missing?

My list so far:
https://wordpress.org/plugins
https://apps.shopify.com
https://www.wix.com/app-market
https://citrixready.citrix.com/category-results.html?f1=applications
https://apps.3dcart.com

Any ideas?