To add to Imagine's list:
Cake flour:
1 cup cake flour = 1 cup all purpose flour, subtract 2 tbsp of the flour, add 2 tbsp of corn starch, sift together 5 times, then use.
Confectioners Sugar = regular sugar run through a food processor.
I use a lot of dried herbs in place of fresh. You can't always get away with this but dried parsley and dried celery work well in soups or anything cooked. Dried dill works pretty much anywhere you would use fresh. Rosemary is pretty good dry. Dried basil, thyme, and mint are less successful.
Use turmeric (super cheap) in place of saffron (crazy expensive) in most recipes.
All oatmeal is instant oatmeal. Buy regular rolled oats and stir boiling water into them. They are instantly ready to eat - do not need to buy the instant packets. I put cinnamon and raisins in mine and it costs next to nothing.
I use extracts in TONS of things and they save me money AND trips to the store. Right now on my shelf I have the following extracts: vanilla, peppermint, orange, mandarin orange, almond, lime, hazelnut, peanut butter, cranberry, blackberry, raspberry, strawberry, pineapple, peach, and maple. I use them constantly - I buy plain tea and add these instead of buying more expensive fruity teas. I flavor my hot chocolate and coffee with them. I bake with them, cook with them, and stir them into yogurt. I make sauces with them, I even do home crafts with them.
Make sauces at home. I started a Homemade Condiments thread last week that should be easy to find - I make mustard all the time, and salad dressings of various types.
I make homemade bread, tortillas, pasta, etc when flour is cheaper per pound than ready-made (which is NOT always the case).
Homemade salsa (both fresh and cooked) are cheaper than purchased and super easy to make. Things like lemon curd and pudding are easy to make at home too and cheaper than buying.