Check out these sites that are offering sign up/referral credits!
Gaggle of Chicks Get $10 for signing up. If you refer a friend get another $10 when they make their first purchase!
Deal Pulp Get $3 when you sign up and another $3 for every referral-no purchase required!
Plum District Get $5 when you sign up and $10 for every friend you refer!
Dealeebob I just learned about this one! Get up to $15 for every friend you refer!