eBuddy XMS is a free messaging app for Windows Phone 7 that allows you to exchange text messages via the Internet.
To set up messaging with XMS, you need to enter your phone number, and then an activation code (sent to you via SMS). You'll be issued with a PIN number once you've entered the activation code. You can then chat with anyone on XMS, adding them as contacts by entering their phone number or PIN number. There's an option to scan your phone book or Facebook friends for any contacts who are using XMS.
Disappointingly, unlike the iOS, Android and BlackBerry versions of XMS, the Windows Phone version doesn't allow you to attach photos or videos, although it does allow you to receive multimedia messages from users who have sent you stuff. On a more positive note, XMS for Windows Phone includes a group messaging feature.
In our tests on a Samsung Omnia W, XMS crashed a few times, and hopefully these stability issues will be fixed in a future release.
Providing you can convince your friends to switch from Viber or WhatsApp, XMS makes for a simple, free messaging app, although this Windows Phone version is quite limited.