My all-time favorite is coupons.com.
Mypoints.com is an awesome way to also print off coupons and earn points to get free gift certificates. With Mypoints you can also read emails and take surveys for points as well. When we switched insurance companies, we used the insurance search feature it has and submited a copy of our policy and got 5000 points for doing so. You can do this every 6 months... so 2 times a year. PM me for an invite!
Swagbucks.com is what I use to search the internet (it's powered by google) and earn "money" for my searches and turn these into giftcards as well.
A new site to shop for household needs is alice.com. This site always has free shipping and in-site coupons you can grab to make for some pretty cheap things!