Humble Bundle has an android app that lets you download APKs, and then prompts you to install them. I've used that to install several android games in the past,

Humble Bundle has an android app that lets you download APKs, and then prompts you to install them. I've used that to install several android games in the past, even for things that are available in different versions on the Play store. It's never showed me pop-up ads, and it's only for installing things that I've already bought via their website. It's not a large store, I expect, and I don't often use it install things, but it seems to be an unambiguous example of something that benefits users. I bought things via Humble, and then am able to install them on any android device I ever have (in theory). If they were to add the ability to actually buy things in the app, it would effectively turn into something with an interface like the Kindle app (which shows me both my library of books, as well as a shopping interface).