Viber is a multiplatform application that allows you to send messages or make calls to users who have installed the same app on their devices; Viber is already widely used in the iOS and Android.
Here's the description from the WP Marketplace:
Viber is available for Android, iPhone ®, BlackBerry ® and now for Windows ® phones.
We are always introducing new features, platforms and possibilities.
Viber is Completely free with no advertising, and we value your privacy: read our policies at http:/ / www.viber.com / privacy
Get it, you'll like it.
WHY Viber ***? ***
* 100% FREE: Viber and all our features are completely Free, with no subscriptions or purchases. Avoid the mobile phone "bill shock" - text other users, on any network, as much as you want. All you need is a 3G or Wi-Fi Internet connection, and friends who get Viber too!
* SIMPLE TO USE: Viber just works, and lets you connect with your friends Whose numbers you Already have. It synchs with your mobile phone number and contact list to show who Already has Viber Instantly. Never again must you seek out your own contacts, create new usernames, or build new lists.
* VOICE COMING SOON : We promised That as soon as the full version is ready, we will make it available I Will Be Able to use Viber Also for Windows phone to make free calls to any other Viber user, anywhere in the world.
The icon above leads directly to Windows Marketplace, and here is the link to BlackBerry App World .