One program I highly recommend is MyPoints.com.
This is how it works: You sign up, and they email you advertisements. You get points for reading the ads, and then you can redeem your points for gift certificates to different merchants such as Blockbuster, JC Penny's, Barnes & Noble, and many other well known stores. So far I've redeemed my points for a $25 certificate to Blockbuster, and a $50 certificate to Home Depot. Not bad for just reading email.
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