FaceTime. It's an odd fish. You can video-call your friends, if they're using a new iPhone 4 (or latest iPod Touch) and you have wi-fi. But you have to call your friend first - over the normal mobile network - and get talking, then switch to FaceTime by activating video. That means if you've got wi-fi but not mobile reception - like on many planes - it can't be done.
Until now! FaceNow is a new app, available now in Apple's app store. It lets you call a contact via FaceTime without calling them normally first. Just open it up, add the contact, click call. Simples!