WhatsApp came along gunning for BBM's mobile messaging crown and it sure has done a good job of annihilating the competition. Aided by its multiplatform support – it's available for iPhone, Android, BlackBerry and Windows Phone 8 – this app's killer features include messaging individual friends or holding group chats that can include photo, contact, location, audio message and video sharing.
It's so quick that WhatsApp has more of an instant message feel to it than SMS so you might find yourself using it more than traditional texts even for one-to-one chats. You also get read receipts in the form of green ticks and the app automatically sends photos and videos to your Camera Roll or Gallery.
Even better that's all free because everthing's sent over your data connection, though we'd recommend trying to use it over Wi-Fi to avoid big phone bills unless you have unlimited data. The app itself is free for the first year then it's US$0.99 a year after that on Android and Windows Phone whereas on iPhone you pay a one-off fee of $1.28.
UPDATE: Some Windows Phone 8 users have reported problems such as sending video and emoticons over WhatsApp so look out for updates to fix these teething issues – the app went live in December 2012.