Apple's App Store for iOS dominates people's perceptions whenever you mention the phrase "App Store". But it's actually just one member of a much larger set of "app stores", most of which don't use that description, but all of which are used to distribute applications to specific audiences.This is particularly important to keep in mind as it's likely that new versions of major operating systems like Mac OS and Windows will incorporate desktop app stores for the first time.
But wait — how could there be many, maybe even dozens, of app stores? Because they often take forms that we don't expect, or piggyback on other platform pieces that weren't originally conceived as an app store. To help make the concept clearer, I've outlined a few of the categories of app store that exist today, and collected some initial data about the size of these different app stores. Keep in mind: Many of these app stores serve more than one category, and those lines will only get blurrier in the future. I've deliberately erred on the side of stretching the definition of "app store" because I think it's very likely that new contenders will rise in areas that weren't previously considered competitive in this space.
- Mobile: These are the most familiar model of app stores, catalogs of mobile applications for phones and, more recently, tablets. As the most mature type of modern app store, they include really simple and robust payment services, easy ways for developers to submit apps, and widespread use amongst end users who have appropriate devices. iTunes leads the way here, of course, with Android being the other major player.
- Consoles: Gaming consoles and video set-top boxes have pretty mature app stores as well. Whether it's Xbox Live, Nintendo's virtual console, the coming Google TV and presumable Apple TV stores, or even the mini-applets that show up on Tivos, Rokus, Boxees, and some smart TVs, these are a relatively familiar form of app store for tech consumers as well.
- Desktop: The most obvious, and most glaringly under-developed category of app store. Interestingly, Microsoft launched a catalog of "Designed for Windows" applications as long as a decade ago when Windows XP came out, but today that's largely farmed out to CNET's Download.com and can't be considered a true app store. Windows 8 is pretty clearly going to integrate something more formal along these lines, and I'd be surprised if Mac OS X doesn't as well, perhaps as soon as in version 10.7. Meanwhile, Steam is extremely successful in being an app store for games on the desktop, and Linux users have long taken for granted a seamless app store experience from their preferred distribution's package manager tools. Some new platforms like Mozilla's open web apps will likely run across the desktop and mobile platforms like tablets.
- Servers: On the server, it's a strikingly different story than the desktop. While there are certainly popular package managers as on Linux desktops, it's increasingly common that server applications are delivered as complete virtual machines, or appliances. Amazon EC2 AMIs, Google App Engine apps, JumpBox images, and of course VMWare appliances are all extremely popular methods of deployment. The payment infrastructure for these app stores is also robust, not surprising given the significant amounts of money spent on server applications and the attractiveness of enterprises as customers. Some server app store deployments don't follow this appliance model, as in the common CPanel tools on shared web hosts, or Microsoft's excellent, but under-recognized, /web tools.
- Libraries: This is the geekiest category, mostly the domain of developers. From Pear for PHP or CPAN for Perl, from code distribution systems like Freshmeat or Macports, or even including old standbys like RPM or synaptic, tools that have long been thought of as mere developer infrastructure for installing library dependencies in the open source world seem poised to mature into relatively full-featured app stores, perhaps at the behest of commercial firms interested in introducing a business model and a more polished experience in front of these workhorses of the Internet. It's easy to picture a Sourceforge or Github creating a simple experience for the subset of the projects they host that can actually be used by end-users. One could argue that plugin installations systems such as the WordPress plugin directory are starting to become this sort of app store already.
So, let's grant that all of these previously-disparate categories of software distribution are becoming simpler for end users, providing a more seamless and integrated experience on all their target platforms, and are introducing more accessible ways to pay for the products they contain. This lets us start to form a picture of the "app store" space as a whole, and a good starting point for creating a strategy to target across all of these app stores is to count the number of apps they claim to contain.
Note: These numbers are rough estimates, and admittedly inaccurate, due to much of the source data being self-reported by the owners of the various app stores. I wanted to post this data here as a starting point for a conversation, so that anyone with more reliable data, or any suggestions for additional app stores to be included, could submit them here in the comments.
That being said, here's a quick overview of what some big app stores list in terms of individuals apps they make available.