BlackBerry Messenger
If there's one thing that BBM has over iMessage is that it has incredible depth. It's simply dense with features, and has its own separate application that's usually preinstalled on the BlackBerry. It's very much like other instant-messaging services where you can add contacts and create groups. Indeed, sending out a broadcast message to a group via BBM is how the London riots were organized. Group communications isn't limited to just messages either -- you can share photos, lists, calendars, and chats within a BBM group. It's also a little like Facebook and Twitter, where you can update your status, change your profile avatar, and let your friends know what song you're listening to.
If you don't have the app...