This SMS reader is great for biking, running, or driving. Unlike other applications, this will only read the messages if your headphones or bluetooth are on. You never have to worry about forgetting to turn it on or off, never any embarrassing messages over the loudspeaker. It will also automatically mute your music while the message is read so it can be heard clearly.
For more control you can have the application stop reading messages temporarily with a widget, or tell it to always read regardless of whether headphones are plugged in in the settings.