I believe this to be the best implementation of its kind. Uses a service instead of an activity context like the rest. The voice will make it so you don't have to turn on your screen, and the sound basically replaces the notification sound. In the end you'll save battery / not have to get up / not have to take your eyes off the road etc.
Speak SMS for Android by Ryan Mattison.
By allowing the user to set options that will either speak the sender of text messages, body or both, you can make a hands-free decision whether it needs immediate attention.
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Update: Added settings button.
Keywords: SMS, Text, Voice, Text to speech, Texting.