So many websites have sections devoted to maintaing a well stocked pantry with specific ingredients and I agree that there are things that in a perfect world you would always have on hand.  I consider the things I always have around to be staples, basic ingredients that you can build a meal off of, but I like to spice up these staples with ingredients I have just heard or read about or something I just ate at a restaurant or friends house that was delicious or something I see at the grocery that looks great, and I consider these dynamic ingredients to be just as important as the static staples.   To this end I am going to list what I believe to be staples on this page and update it when I find a new dynamic ingredient that I am really loving!

Pantry Staples - some of these are obvious and somenot so much, but I find them all quite useful:
brown sugar (light and dark)
cocoa powder
baking powder and soda
2 varities of each short and long pasta
rice and/or couscous
olive, vegetable and canola oils
bread crumbs
mixed nuts
onions, potatoes, garlic, tomatoes (I try to buy these in the bulk section at the grocery to save money)
ketchup - some people say this belongs in the fridge but I don't like cold ketchup
1 can each of: diced tomatoes, tomato sauce and whole tomatoes
at least 2 varities of canned veggies - whatever you like, for that day you don't have time to go get fresh
cream of mushroom and or cream of chicken condensed soup - this is from the southern casserole maker in me

Spice Staples - whenever I walk down the spice aisle there are so many I want, but these are the always haves:
garlic powder
minced onions
bay leaves
paprika (sweet and smoked)
any brand premixed grill seasoning

Refrigerator Staples
worcestershire sauce
soy sauce
tube of tomato paste
dijon mustard
whole grain mustard
pepper jelly
cheese - slices, blocks, shredded, whatever you like
jar of olives
queso fresco - delicious mexican cheese, keep a round of it at all time!
sour cream

Dynamic Ingredients to spice up the staples
dried cranberries - I got on this kick last summer and now love them to mix in with cereal in the mornings or a couscous salad or even in some desserts
tortillas - this all started when I bough a package that was too big, but they are so versitile I love them!

It was a big expense to get all of these things the first time, but now that I have stocked my kitchen once keeping it up is easy because things don't usually run out at the same time.  To help yourself keep up with what you need or want invest it a cheap note pad with a magnet on the back, stick it on your fridge and then when you run out of a staple write it on the pad and then take the list with you each time you go to the grocery, but once you are there keep an eye out - if one of your staples is on sale go ahead and buy it.  You may not need it then, but you will need it eventually and when it will be in the back of your pantry and you will have saved money!