If you order from Drs. Foster & Smith (sell aquarium, pond, and animal supplies) or maybe want to at some time, they have a daily chance to win $100 for the next 25 days and then $500 for 5 days. You can opt out of e-mail ads if you want and still enter. No purchase required. It is a pain though that you have to fill it out every day separately.

DFS Sweepstakes