How to make $100 last all week?

The only thing that comes to mind for me is to buy off brand foods, they are usually cheaper.

you can get water from a tap

Scour the ads and budget accordingly.
I used to live off of frozen chicken breasts and frozen veggies.

How many are you cooking for? Beans and rice go a long way. I buy eggs often, as they're cheap and full of protein. You have tortillas, too, so you could make veggie tacos with beans, rice, corn, and tomatoes. I like to add spinach to mine and maybe some cheese and onions (both cheap). Tostadas with veggies and beans are good, too.

Often, I'll just choose the meat on sale. I got some frozen tuna today that was pretty inexpensive and delicious. Maybe having a protein bar for breakfast or lunch. I also buy cheaper produce; I stick to things like bananas and carrots rather than berries, for example.

I don't know, but if you can get $100 of groceries to last all week I want to know the secret!! We usaully spend $150 per week for our family of 4, but that also usually includes other essentials like toiletries and cleaning supplies.

Our main staples are
milk
chicken
ground white turkey
frozen vegetables
fresh fruits
Cereal for the kids
Oatmeal for me
peppers
onions
whole wheat bread and/or sandwich thins
eggs
low fat cheese

In order to save money on groceries and cut down on un-needed items we buy generic, check coupons and sales ads, and try to plan 3 or 4 big meals for the week then work off of left overs for the rest of the week.

I'd go with eggs and oatmeal.

I have only $100 for groceries for 7 days. I already have:
Butter
Cooking spray
Corm meal
Splenda
Water
Frozen corn, peas, carrots
2 bags dry beans
3 bags brown rice
Cooking spinach
Mashed potatoes
Tortillas
Diced tomatoes
Tostadas

... Basically the foods I dont tend to eat because I have other foods.

Any how, ideas for affordable foods that will last? Any ideas at all!

Beans and Rice are supercheap, but they end up tasting bland unless you season them up well. One onion and a small head of garlic won't kill your budget, but they will make your foods taste better. I've also made beans and rice with stir fried pork and some kind of fruit like pineapple. But stick to peaches or plums,as they are in season now. Veggie stir fry with lean pork, chicken or fish is always a good bet, and you already have the rice. For lunch, I'm a big fan of the meal size salad, with some kind of protein, like packaged tuna. For me it is worth it not to have to drain it. Is your "cooking spinach" frozen? If it's fresh, I'd use with the tuna and orange wedges as a salad. If the spinach is frozen, I'd add it to an omelet. Finally, use the corn meal to make polenta as a side dish. You can make it savory with herbs, or sweet for breakfast or dinner.

So, here's what I'd add to your shopping list for one week:

1 vidalia onion ($.99/lb here in PA)
1 sm head of Garlic (@ $1.29)
Boneless pork chops, 1 lb (7.99) (about 5 3-oz servings)
Various fresh vegetables, about 1 1/2 lb (4.00)
1 lb peaches (1.59)
1 3 lb bag of boneless, skinless chicken breasts (5.97)
1 bag of hearts of romain (3.99)
1 navel orange (1.00)
1 doz eggs (1.89)
Other fresh fruits and veggies
Whole wheat bread (4.00)
Skim Milk (1.89)
Skinny Cow Ice Cream bars (3.99) cuz everyone needs a treat

Total: 38.59 plus the cost of other fruits and veggies

Stick to in season fruits and veggies, and stay away from most prepackaged foods. Good luck and good eating!