While in a call it will switch automatically between speaker and head output based on the values from proximity sensor.
Have your phone on the ear and it will be like normal but take it away from your ear and it will automagically switch to speakerphone and it even includes a conference mode that if you place it upside down on a table it will stay on speakerphone.
It checks if a wired or bluetooth headset is connected and will deactivate itself in this condition.

This will only work if you have a phone with proximity sensor hardware like the nexus, Desire, Samsung Galaxy S, G2, DroidX.

If it doesn't work on your phone, please contact me by mail

Screenshots: