Even though the iPhone can already make calls, wouldn't it be nice to ring someone without using up any minutes? Carriers probably wouldn't be too excited about such software, but who could stop developers from creating those programs?
According to Steve Jobs himself, only "porn and malicious programs" are not allowed on the service. I can smell VoIP (and lower monthly bills)... Reasonable price: £5 [Update: looks like it's only on Wi-Fi - Ed]
2. Improved virtual keyboards
Say what you will about the beauty of the iPhone, but few would claim that its virtual keyboard is perfect. Wouldn't it be great if other companies took a shot at creating virtual keyboards that went into landscape mode on the Home screen or had slightly better accuracy functions built-in? I'd buy that app in a second. Reasonable price: £13
3. Mobile Office Suite
Let's face it - the fact that we can't open, edit and save Office documents on the iPhone is ludicrous. But now that Apple has opened the device up, I'm looking forward to the first real Office suite that lets me do just that. Until then, the iPhone isn't nearly the enterprise phone some are touting it as. Reasonable price: £20