Skype is one of the most widely used messaging and calling applications available on practically every current platform. Skype for iPhone is, as the name implies, the iOS port that works on iPhone and iPad. Available for free from iTunes or several download sites, including the publisher's, Skype installs and configures easily.
Skype lets you message or chat with anyone on your contacts list. You can also set up either audio or video conferences easily with Skype, as well as conference calls, by hitting a single button after choosing the contact. Calls over standard data networks, from one Skype device to another, are free, and there's premium services for calling landlines for a monthly fee. Skype's call quality is very dependent on both ends' connection speed and reliability, and while the quality is often excellent, there are times it deteriorates with the data rates. In our testing, most Skype calls were as good as standard voice calls, and setting up a conference call is simple and fast. You can send attachments (photos, audio, etc.) using Skype.
We've used Skype for several years on many different platforms. Skype for iPhone has a nicer interface than some of the desktop versions, and it is easy to learn to use. Despite having multiple messaging and calling apps on our iPhones, we end up using Skype the most simply because of the massive install base and the ready availability of clients.