Tango Video Calls (free) was just released today and seems another good option in addition to FaceTime and Skype. Calling is free, and you can connect with other iPhone 3GS or 4 users, as well as with Android phones. It works over 3G and Wi-Fi. You can invite friends and others via one-click email or text message. The app automatically imports your iPhone contacts. It lets you easily switch to audio only, and to swap the large and small screens so that you can see more clearly what the other person is seeing. You can also easily switch between cameras on your iPhone, so that the person you're talking to can see the environment rather than your image. The app requires an iPhone and apparently won't work with the iPod touch.