You can earn 10,000 Starpoints per referral to the Starwood Preferred Guest Credit Card, up to 55,000 Starpoints per calendar year. That's double the normal Starpoints per referral (but the same annual cap.)

I value 10,000 Starpoints at $250 and 55,000 Starpoints at $1,375.
How does the referral bonus work?
Look for an email with the subject "Limited Time: Refer a Friend, Earn Double the Starpoints" and click the Refer Now button.