Just out today, Thrutu (free) seems an amazingly useful app. You can actually share data and media with someone while you're talking to that person on your iPhone. It works with other iPhone users — and also with Android users. You can share your location such that you see each other's position updating live on a map. You can snap and share a photo, or send one from your gallery. And you can send contact information such that contact details appear instantly in the other caller's address book. One feature is perhaps more playful than functional: you can vibrate the other person’s phone.