Mac OS X comes with iMessage, Apple's messaging system that's also built in to iOS. While iMessage is great, it doesn't allow cross platform messaging. Enter Viber: an application for Windows, Mac and Linux that does exactly that.

Viber is free software that provides two key functions. There's text chat, much like SMS messaging, but with the ability to send images too. Then, there's voice calling; Viber boasts high definition audio for superb quality and clarity.

There are plenty of messaging apps besides iMessage and Viber; Skype covers the same market and has a massive reach. However, Viber allows users to transfer a call from one device to another - a superb feature that its competitors inexplicably lack.

Other benefits include:

• Easy Viber connection using an existing phone number, making signup really simple.
• A range of graphics - named Stickers - that can be sent between contacts for free
• Complete cross-device support for Windows, Mac and Linux (Ubuntu).
• Free calls and messages to any country in the world.
• Mobile apps to continue the conversation on the go.

With support for wifi and 3G and an ad-free interface, Viber is quickly gaining pace. For calls to regular phones, try its add-on service, ViberOut.