Xbox.com has for its Gold Members (New & Existing): 1-Month of Membership for Free. If you are an existing member, you can extend your Xbox live membership by a month!
When you log in you will see the following offer: "Gold 1-Month Free Trial" even if you have an existing account. If you have used this offer previously then you may not qualify again.
Note: If you do not wish to continue your membership after the trial then remember to turn off the auto renewal.