Skype

Imagine being able to use your iPhone to make phone calls! (Har har.) Now that Skype works over 3G and can take advantage of iOS 4's multitasking capabilities, it's actually a viable alternative to using your mobile minutes. Even if you don't want to go so far as to replace your regular phone, it's still a great tool for making international calls on the cheap and chatting with your Skype contacts on the go.
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