There's a lot of buzz about iPhone IM clients.Other applications have gotten a lot of coverage for allowing iPhones to chat from their AOL, Google, ICQ, Jabber or a host of other screen names.
WhatsApp [iTunes Link] is different, calling itself an "iPhone-to-iPhone chat application" that provides direct messaging between iPhones. The application features push notifications and provides functionality similar to SMS.
So why bother trying it? The app makes it easy to chat with other iPhones with no need for screen names, logging in and out or the added cost of text messaging and cellular charges. It's a lot like an iPhone version of Blackberry's popular messaging service. You'll also quickly and easily be able to point out which of your contacts have WhatsApp installed. Even if you're offline, your messages will be saved until you're back online.
Other cool features include the ability to "Broadcast" or send a message to multiple users, emailing of your chat history and sound and visual badges to show your unread message count. Users can also see when their friends are typing as well as the last time they checked their messages.
WhatsApp is free "for a limited time" so head on over to the App Store and grab it.
Here's a few shots of the app in action: