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.
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.
Another thing WhatsApp does well is group messages. You can use the Broadcast Message feature to bring up your entire list of contacts, then click radio buttons to quickly add contacts for a message blast. There's also a New Group feature, which lets you add contacts for a group message much as you would in iMessage or another chat app. But you can also look back over the group messages and list just the locations or just the media added to the group chat by all members. These are not amazing or groundbreaking features, but they are a bit more than what you can get with the standard messaging features on most phones.
So the real advantage of WhatsApp Messenger is the money you save when communicating with Android (if you use iMessage) or while communicating internationally. In other words, with WhatsApp Messenger, and a little cooperation from your most-texted friends and family, you could easily get the lowest-cost texting plan with your carrier and save some money every month on your smartphone bill. It shouldn't be that hard of a sell either, since your friends will save money on their phone bills as well.
A note about pricing: At the time of this writing, WhatsApp Messenger is currently 99 cents for iPhone users, but free for Android users. But if you look through the app's history at the iTunes store, there have been times when it was free and times when it cost 99 cents. On Android, you get a different experience. It's free initially, but after a year of use, you'll need to pay a 99-cent yearly subscription fee. According to this TechCrunch story, WhatsApp will bring the 99-cent subscription to iOS sometime in 2013. This is the first time I've seen such a pricing scheme, but it's hard to argue with only $1 per year.
Overall, WhatsApp Messenger is not much different from the text-messaging services you get on your smartphone, offering a few neat extras that make it a little more fun and useful. But if you do a lot of texting, this app is a must-have, giving you unlimited free texts with all your friends, whether iOS- or Android-using, without having to worry about extra charges, even internationally.