HeyWire (see website) is a cross-platform app that enables users to text each other for free from the US and 45 other countries. It sends SMS over any internet connection from iOS, Android, "dumb phones," and now, Windows Phone.
"We are very excited to provide the only free texting app in the Microsoft Marketplace -- a platform that Gartner predicts will capture the number two smartphone spot by 2015," said HeyWire founder and CEO Meredith Flynn-Ripley. "This was an important service for our company to bring to consumers, which continues HeyWire's mission to facilitate free global communications amongst groups of friends. Now, WP7 users can send and receive text messages with friends on multiple devices, quickly and easily."
HeyWire not only lets you connect across mobile platforms, but Twitter as well. It has a simple, straight-forward user interface and can be downloaded for free here. Thanks, Amir, for the tip!