Version 3.0 of Skype (free) just came out yesterday and now offers video calling over both WiFi and 3G. Skype has long been the most widely used service for Internet telephony, and now you can use the iPhone 4 front-facing camera to conduct video calls. You can read a great hands-on review on the PCmag.com website. The quality is reportedly quite good, even over 3G. And you can call cross-platform, even to PC users.