It can save you money. It can make calls where AT&T's (T) signal is weak, like indoors. It can turn an iPod Touch into a full-blown cellphone.
And it can ruin the sleep of cellphone executives everywhere.
Line2 gives your iPhone a second phone number -- a second phone line, complete with its own contacts list, voice mail, and so on. The company behind it, Toktumi (get it?), imagines that you'll distribute the Line2 number to business contacts, and your regular iPhone number to friends and family. Your second line can be an 800 number, if you wish, or you can transfer an existing number.
The Line2 app is a carbon copy, a visual clone, of the iPhone's own phone software. The dialing pad, your iPhone Contacts list, your recent calls list and visual voice mail all look just like the iPhone's.
Line2 also turns the iPhone into a dual-mode phone. That is, it can make and receive calls either using either the AT&T airwaves as usual, or -- now this is the best part -- over the Internet. Any time you're in a wireless hot spot, Line2 places its calls over Wi-Fi instead of AT&T's network.