I liked Viber when I tested it several months ago. It's an iPhone VoIP app that allows free phone calls to any phone that has the app installed. I thought the terrific audio quality, lack of ads and ease of setup were all positives.
Viber has been updated today to version 2.01 with free text messaging to all your Viber contacts. There are also notifications for incoming text messages, and the app has some small GUI improvements. Although Viber currently does not support the iPad or iPod touch, the developers say that is "almost here."
Viber Media claims the app has had more than 10 million downloads. The app requires iOS 3.1 or greater. There will be Android and BlackBerry versions, but those apps have been promised for several months and are no-shows so far. The app works on both Wi-Fi and 3G networks, and best of all, it's free.