Not sure how long this will still work for but I did it less then 5 minutes ago and got my free Gold^^ just thought I'd pass it along to everyone else who might need it.
You MUST have a free membership currently for it to work.
You MUST have a Valid payment method (It will NOT charge your account, Just make sure you turn off auto-renewal)
Next month I will have been a member for about 2 years, and only payed $2 so if anyone finds more of these links please keep em coming!
Games with Gold
August 1st-15th - Crackdown
August 16th-31st - Dead Rising 2