the only way for the average person to get an app onto the iPhone that isn't in the iTunes App Store is for the app to be web-based (e.g., html5). and, then it's not technically an app as much as it's just a webpage. Google does this for Buzz, for example. i save the webpage as an app, but it's not the same and i don't know of anyone who creates an app this way to make money. if you want to make money then you go through the app store as that's where 99.87% of iPhone users get 100% of their apps.
Also check out appbrain.com. They have an app you install, then you can browse their website from a desktop and select apps you want which are then automatically pushed and installed on the phone. The website also syncs to your phone and lists apps as already installed, or ready for updates. It's pretty nice browsing for apps in a full size browser.
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