Marketplace and Platform Adoption

I am spending a couple hours today going through popular app marketplaces looking for document management and collaboration applications. I'm also comparing each marketplace as browse through them.. I'm looking through four primary marketplaces: I find applications from companies I've never heard of. It seems like a new way to discover tech products. I'm also seeing applications from big providers like, eventbrite, etc.

I'm thinking about the marketplace concept and how they influence my adoption of these applications:
  • Google Apps Marketplace - I've installed a couple apps, but primarily if it isn't a Google offering I don't notice. See potential tough.
  • Chrome Web Store - Brand new...just checking out.
  • Salesforce AppeExchange - Interesting offerings and platform, but I am not a daily Salesforce user or developer.
  • Evernote Trunk - I am dedicated Evernote user, but really haven't found anything here I would use. I see lots of good things, just nothing I'd use.
There is lots to think about when it comes to application marketplaces: Which ones we should pay attention to? Where we should consider building apps? Where we should consider partnering with existing players?

While doing this research I also searched Google for more "application marketplaces": I then went through about 10 pages of Google results. A couple other no name marketplaces. What other top marketplaces am I missing?