I don't normally take cabs but used one recently and on the bottom of the receipt it offers $20 off first ride with the app Rydo.
Downloaded app, signed up and added credit card to receive credit. Haven't used it yet and not sure if it expires but appears to have been applied.
Edit: received an email after signup (used the promo code in the signup process..) suggesting to use the code 'TAXI' for the same $20 first ride discount but as expected it won't allow you to double up with both.
Welcome to OzBargain!