When the iPhone 4 was introduced, we were all shown FaceTime. FaceTime was Apple's new video conferencing software that made it's debut with the iPhone 4 and was later carried over to the iPod Touch 4G and Mac OS X. While the video quality is definitely better than the other Apps we've seen, the software is severely limited by its inability to work over 3G.
Skype is the most recent App to join the iPhone video chat lineup. It's also one of our favorite for the simple reason that it's so widely used. It's also nice that it can be used across various platforms, meaning that an iPhone user can talk to a Mac user or a PC user.
This was one of the first Apps of its kind to become popular for iOS. One of its strongest features was its ability to be used on both Android and iOS, meaning that users of both operating systems could video chat with each other.
Tango is a great choice for iPhone users because it offers the openess of Fring (iOS to Android) but has easier to use. Setup is much simpler, and that makes it a great choice for families trying to keep in touch, especially when one member may not be as tech-savvy as the other.
4) Yahoo! Messanger
Like Skype, this application is nice in that it's widely available to users on several platforms. It's not uncommon for a person to have Yahoo! Messanger on their computer, and that minimizes any problems with having to download and install new software.
This solution requires a jailbroken iPhone, so it's probably not the most accessible. There are plenty of Apps on the App Store that are also available for free that allow for the same kind of use. Still, it's worth checking out if you already have a jailbroken iPhone, and it gives you the ability to talk to users on various mobile platforms.
The Standard for Video Chat
Unfortunately, it doesn't look like we will see a standard anytime soon. Apple did leave FaceTime open source, but it hasn't been used as widely as many of these other Apps. If we had to choose one, it would probably be Skype. That's not because we feel it's the best quality video chat, but we will look past that for the simple reason that Skype is available for everything from the PSP to the PC and Mac and just about every other mobile device.