Here’s what I’m thinking: one of the most popular apps since the inception of the iPhone App Store has been Pandora. Pandora obviously works through the web browser, but plenty of people would be into a small app that sits somewhere on your desktop running in the background. How do I know? Because Pandora actually already makes such an app — but it runs on AIR and you need a Pandora One premium account to use it. What if Pandora made a free ad-supported Pandora Mac app? Or a paid version (maybe $5 or $10) that gives you premium features? A lot of people would want such an app.
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But a real opportunity may exist in small apps that don’t just fully mimic popular web apps, but instead extend upon them. Imagine a Facebook app, for example, that offered a great photo upload and viewing experience? Again, this is sort of the idea Apple seemed to originally have with their Desktop Widgets, but those never really took off. One reason, undoubtedly, is that distribution was lacking, and developers had no way to make money from them. The Mac App Store solves both of those issues.
Mobile apps do a lot of things that would be done in the browser on the desktop. The question is if that trend will make it to the desktop in the trojan horse of the Mac App Store. I think Siegler may be on to something — a whole class of little apps that make life a little bit easier.