WhatsApp is undoubtedly the grand daddy of chat applications present on SmartPhone OSes these days. It was the one that truly generated the unprecedented numbers - 17 billion daily message transfers and 100 million users on Android alone.
As many of you might know, in the beginning, it started off as an iPhone-2-iPhone only messaging app but after it recognized the value of Android and other Operating Systems, it realized itself as the first major cross platform free messaging solution with presence on iOS, BlackBerry OS, Android, Windows Phone OS, Symbian and Nokia S40 as well. On every system other than iOS, its initial download didn't cost money but user has to pay $1 for each year of usage but on iOS WhatsApp still kept the $1 Buy-it-now model instead of the subscription one. Now it looks like they have decided to jump off the Buy-it-now wagon from iOS as well.
Its not clear as of now if and when WhatsApp Inc. is going to adapt to subscription model on iOS but for now they are offering WhatsApp free for all iOS users without any deadline for its availability. Also, with the latest release of WhatsApp users get to back up their messages on their iCloud account including all media files (except videos) and they also get the privilege to send multiple photos at the same time. So iOS users can rejoice and not worry about the $1 payment for this amazing application which has started to become a need for some rather than a luxury.