Editors' note, February 19, 2014: This review has been updated with information from Facebook's announcement that it acquired WhatsApp.
WhatsApp Messenger (iPhone|Android) lets you chat for free with anyone else who owns the app and offers a few extra useful features to make text messaging more fun. With this chat app installed on your smartphone, you'll save a little money on text charges through your carrier and across platforms. The only catch is that you'll have to persuade your most frequent texting buddies to download the app, too, but once they see WhatsApp's advantages, they'll probably give it a try.
WhatsApp Messenger was recently acquired by Facebook, and while that doesn't mean a big change to the app in the near future, it makes me wonder how Facebook plans to use it. Whatsapp is already incredibly popular around the world, so any news about changes to the app will be watched closely. Should we expect WhatsApp to continue in its current form alongside Facebook's own separate messaging app or will Facebook's offering suddenly have the same features, as Whatsapp rides off into the sunset? It's hard to know at this time, but check back here for more information and an updated review when any new Facebook chat app becomes available.
Like iMessage for iOS, WhatsApp Messenger shows your chats in little text bubbles, gives you a time stamp for messages, notifies you when a text has been viewed by your recipient, and lets you include photos, audio notes, and videos within your conversations. But with WhatsApp Messenger you can also change your background and send your GPS location to an interactive map. You can use premade away notes such as "I'm busy" or "Available" or even "My battery is about to die," and you can block specific contacts from within the app. You also can easily send a friend's contact information to another user without leaving the app. So, while the app is not leaps and bounds ahead of other messaging apps, there are some nice extra features here.