Here is my list of things that you should always have on hand. They are all multi purpose, inexpensive, and will help you make meals without having to go to the store.
The very basics
Salt and pepper- duh, but go for sea salt and peppercorns in a grinder
Vinegars-balsamic and rice wine
Red pepper flakes or cayenne powder
Dried herbs- rosemary and basil to start with
Other spices you like- ginger, paprika, cinnamon etc.
Honey- unless you bake, you can use honey to sweeten everything
Soy sauce or tamari
In your cupboard
Pasta- this should also include some ethnic varieties like soba or rice noodles
Rice- buy in bulk for pennies at an Asian grocery store
Beans- kidney, black and cannellini
Cornmeal and/or panko
In your freezer
Gnocchi- for when you don't want pasta
Frozen veggies- more important in the winter
Nuts- unless you're eating them right away, store in the freezer. Peanuts and pine nuts are staples
Tofu- prep first by squeezing the liquid out
In your fridge
Condiments- a good mustard is a must have
Then fill in with fresh and seasonal ingredients. I'm lucky enough to have farmer's markets I visit every Wednesday and Saturday in addition to my CSA pickups every other Monday. This summer I've realized that my fridge is always brimming with fresh produce, sometimes more than I can eat. I think my pantry items and strategies will change significantly when it gets colder and fresh ingredients are harder to come by. I guess this is why so many people are canning and freezing fruits and vegetables. Anyways, having a well stocked cupboard will make cooking a lot easier. Even if you have to visit several different stores, shop around until you know where to get your pantry items in bulk for cheap.
Are there any items I missed that you always keep on hand?