Starbucks Free Star Codes to Try:
MCATALINM - 5 Stars
MCATALINH - 5 Stars
MCATALINL - 5 Stars
MCATALINK - 5 stars (added from comments)
If they don't work for you today try them again each day over the next few days they appear to have a limited amount of times they can be used per day!
3 things you need to know:
1 - You must have a starbucks rewards account, it's free but you need to set it up.
2 - You must have your name and address in the account for the codes to work.
3 - The codes cap out during the day at some point so if they don't work today don't assume they won't work tomorrow.