One of the main things any good cook knows is what a difference a pinch of this or that can make to the overall flavor of a dish. The kinds of spices there are is nearly limitless, especially if you're willing to drop some serious cash on things like saffron and cardamom. However, the vast majority of dishes can benefit from a few affordable key spices:
Salt (sea salt preferably)
Pepper (white, black, red, whole peppercorn...)
That is as basic as a spice rack can get. If you want to spring for a few more, may I suggest paprika, basil, chili powder, cinnamon, nutmeg, rosemary, thyme, lemon pepper, cumin, bay leaves, cilantro, mustard and Old Bay & Nature's Seasoning. Also it is always a good idea to have some chicken and beef bouillon, much cheaper than buying stock every time!
For those lucky Bowling Green, OH residents, did you know that Ben Franklins in town sells many of these spices for only 0.99?!
(Thanks to Jason for suggesting I mention this!)
So if your spice cupboard is bare, start stocking up, because the recipes will be starting soon!