For myself, I know of two major ones:

Google Play Market
Amazon Appstore

And two others that I get apps from:
Humble
F-Droid

...and it would be helpful to me if, when the Android section gets more fleshed out, these options were included. So far, I have only tried adding the ones from Humble, but there are some non-shovelware ones from Google Play and F-Droid I would like to add. I do not personally still use Amazon, but it is the locked-in store for some devices, so I thought it was worth mentioning.