If you don't mind eating the same meal for several times a day several days in a row I can definitely say you want things like Soups, chilis, big crock pots of meals you can simmer and leave to cook.
Would like to echo every one else's statements about rice, beans and grains and such. Pastas are also quite cheap but you probably don't want to be eating them also the time. If you have a large slow cooker, you can buy tougher cuts of meat for cheaper and put them in to simmer with some kind of sauce or make a soup, it will make the meat really tender and you save a pretty penny!
Also Farmer's Markets are a good one! Not everyone is lucky to have one, but if you do know of one you can get very nice, tasty veggies and fruits for usually cheaper than the store, usually bigger and better tasting and probably with little to no pesticide use (but I'd always ask if you are worried about it).
Also if you have the space (and a green thumb) you can try growing your own veggies and herbs
I think it's even possible to grow some herbs indoors in the winter. Seeds are always cheap and can be found in a lot of places and you can grow exactly what you want provided you have the right climate. With some loving care your veggie yields will probably be bigger than what you could ever find in the store and you might have so much left over you can freeze or give as gifts to friends.
