Appstore. Initially for Android users in the US, this will be in direct competition with Android's own Marketplace.
Seeing as how everyone is making an app these days, there seems to be no shortage of places to sell them in as well. Although, if Amazon's Appstore can't sell to countries that Android's Marketplace doesn't sell to either, I don't really see the point. Hopefully paid apps will be available on Amazon's Appstore faster than Android's own Marketplace in restricted countries like the Philippines.