Here is a list with some of the top SaaS App Marketplaces and SaaS App Stores out there.
I didn’t create the list (I forget the source) so huge thanks to the person/team that did this :)
As oppose to comparing them all, a few interesting things overall are the wide range of industries and verticals (health, finance, HR, etc) and the variation in experience and functionality, from deep integration with the core platform, to more of just a simple app listing or app showcase.