If you jailbreak your phone you can install any apps from Cydia, the unofficial installer that supports multiple sources of unofficial apps. The problem is tha

If you jailbreak your phone you can install any apps from Cydia, the unofficial installer that supports multiple sources of unofficial apps. The problem is that only a very small number of iPhones/iPod Touches are Jailbroken, and you have to deal with purchasing/registration yourself if you're a developer.