This Ping app by the guy who made PushGmail is a mix between text messaging and instant messaging . What you need to know is that it’s free and it’s fast. Ping is designed to look like the SMS app, and behaves much the same way. The messages you send arrive at the recipient’s screen instantly—similar to IMs—but also pop up with a Push Notification if you’re not actively using the phone. It’s basically free SMS for you to everyone who has an iPhone or iPod Touch (over Wi-Fi) as long as you can convince them to sign up for Ping

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Ping is Like a Free SMS Client For iPhone and iPod Touch Users [IPhone Apps]