I am currently out of the US and I was wondering if there is an app that allows texting over WiFi.
You can send text messages for free through Google Voice--everything you need is in the app. Just make sure you change the settings to refresh every 5 minutes so that incoming messages won't be delayed. If you want faster notifications, log in to the Google Voice Website and configure your account to send you e-mail alerts when a new text arrives.
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