Naturally the first recommendation is MyCouponspot. Coupons for $200 in your choice of groceries, and if your grocery store does double or triple coupons, you can save even more!

These free sites offer online coupons which are generally for things like electronics and online shopping. Deals such as a certain % off an order at amazon.com or Staples if you spend 50 or a hundred dollars.
You rarely find groceries on them, but if you're looking for online shopping they're worth a look.

CoolSavings has banners up everywhere so you may have seen their ads. Link

mycoupons.com has forums. Link

Coupon Mountain Link

Eversave.com Link

Flamingo World Link

Slick Deals Link

The Yahoo Coupon Directory Link

And these sites offer freebies, free samples, and other free deals. Again not much in the way of groceries, but free is always worth investigating.

The Free Site is my personal favorite. Link

But others include:

Free Stuff Center Link

Totally Free Stuff Link

free.com

This is a nice site to use in conjunction with mycouponspot.com, just remember to print out the page for wherever you're shopping, since their coupons are only good at particular grocery stores:

Valupage Link