yes, but not all iphone 4 apps are available on the iphone 3gs/3g/2g becuase they are too high resolution apps, or need a front facing camera.
you need an ipod touch or iphone :)
Yes, iPhone does have several apps that function as task organizers. They even offer reminder service.
You would need to have an iTunes account.
No, not all applications are priced. Many useful apps are free.
No, you don’t need another iPhone to install iPhone applications. You only need an iPhone, iPad, or iPod touch that supports the app and an Apple ID to download apps from the App Store. If you mean installing iPhone apps on an Android phone, that’s not possible because iOS apps are designed to run only on Apple devices.
You buy the iPhone app
apple create the apps on iPhone
no you cannot use ipod and iphone apps on itunes, but you can buy the apps on itunes and put them on your ipod and iphone
Although iPads can download apps made only for iPhone/iPod Touch, the iPhone can only download iPhone or iPhone/iPad apps. Apps made only for the iPad do not appear on the App Store. Luckily most of these apps have an iPhone counterpart such as Angry Birds (iPhone) and Angry Birds HD (iPad only).
The good news is that IPhone tracking is available and the ... you can look out for such as the GPS tracking ... iPhone Jailbreak News - iPhone Apps - iPhone ...
If you want to add apps to your iPhone from a computer, the easiest way is to use the App Store directly on your iPhone. Apple no longer lets you install new apps through iTunes like before. If you already have apps downloaded on your computer or need to manage iPhone data, you can use tools like MobiKin Assistant for iOS to manage backups, transfers, and other iPhone content. For apps themselves, though, you’ll usually need to download them from the App Store on your device.