One of the oldest (launched in 2000), and arguably the best, paid survey company around. SurveySavvy pays up to $10 per survey, with most surveys in the $1 to $5 range. They offer frequent surveys and a number of other research based earning opportunities.
Referral Program: Members are paid $2/survey for referrals and $1/survey for second level referrals. The most generous referral program out there.
Update: Check out their new SavvyConnect program. Get paid monthly for installing an app on your mobile device(s).