A slew of applications - both proprietary and platform-agnostic - are available in the market. The best-known is the BlackBerry Messenger, known to all fans of the brand as BBM. Two people with BlackBerry phones can chat for free with each other no matter where in the world they are, and that too, in an encrypted environment.
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But at a time Android devices and other platforms are steadily eating into proprietary smartphone monopolies, cross-platform messenger apps are rising in popularity. These apps are free and unhindered and offer many additional features such as group chat, multimedia messaging and push notifications. The only two requirements are that you need to have a data connection on your phone and your friends need to have the same app installed on their handset.
LiveProfile is totally free too and available for iOS, Android and BlackBerry devices. Besides offering unlimited messaging, it also integrates with social networks such as Twitter and Facebook to figure out which of your friends are already using it. PingChat is available for iOS, BlackBerry and Android. You can send regular text, photos and video messages for free to up to ten people at a time.
Since RockeTalk is available as either a Symbian or Java app, it works on almost any device - including most smartphones and feature phones. It does regular data and photo messaging, but RockeTalk is different from the others because it includes communities where you can browse content shared by others.
Finally, WhatsApp has support for iPhone, Android, Symbian and BlackBerry. There's one key difference between WhatsApp and all the others. The others use a PIN, username or some sort of unique identifier - you have to add friends you want to chat with. WhatsApp reads the address book in your phone and automatically identifies which of your contacts is using WhatsApp - because each account is linked to a phone number and SIM card.
That's also why WhatsApp is not supported on the iPad and iPod Touch. WhatsApp is also the only messenger that's not completely free - you get one year of free usage, after which you pay $1.99 for every year of unlimited usage. Other advantages include support for offline messages location sharing and complete chat history.