It's barely worth it, but the other two I'd consider:

Nook - https://nookdeveloper.barnesandnoble.com/

Nabi - https://www.fuhuapps.com/

There are more though:

http://www.crazyandroid.com/24-android-app-store-alternative...

You can also port Android apps to Blackberry 10 without too much trouble. https://developer.blackberry.com/android/documentation/port_...

I don't know any other standalone /app stores/ - Play & Amazon are the main ones I think.

You could submit to directories that link to those, e.g:- Appseeq lets you add a page for your app that links to your main app store listing on whatever (google play etc..) - so it's more of an app directory, but worth submitting to a few of those too.

I've used those two as well as GetJar and Nook and Samsung Apps. Does it matter, though? Anything other than the main ones tend not to have enough user traffic to matter anyway.