The built-in iOS Messages app
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Pro Can use your computer to type messages with iOS 8
Pro Comes pre-Installed
No need to download or sign up for a new service
Pro Friends don't need to "sign up"
You can just message them using their existing phone number or Apple ID and they will get your message.
Pro Clean UI
Pro Free & unlimited messages between iOS and AppleID users over WiFi
Color-coordinated to show if you are conversing over iMessage or SMS (where text messaging rates apply).
Pro Clear indicators for message delivery and receipt
The app indicates when messages have been sent, when they have been read, and when someone is typing.
Pro Has iPad & Mac clients
Pro Supports voice messages
Voices messages can be sent and listened to within the device