A great deal of what I'll be posting here is Indian cooking, as that's the kind I do most often these days.
Below are my spices:

Some close-ups:

clove

mustard and cumin seed. beautiful!

okay, I know lentils aren't a spice, I just happen to keep them in my spice rack and they add colour to the photo-shoto.

cinnamon bark

ground coriander
There are two categories: whole spices and ground spices. Whole spices are fried alongside the onions from the beginning of the curry. I tend to eschew garam masala; it's much easier for me to fry clove and cinnamon into the onions. It tastes punchier as well, I think.
WARNING!!! fenugreek seed an exception to this; it needs to be fried for VERY short periods of time else it becomes bitter. Generally it should be the last spice you put in. Me, I rarely put it in at all, because I'm so paranoid it'll get bitter. But I'm working on it.
The ground spices are added after the garlic and ginger are just about fried (g&g go in after the onion has softened). One thing to note is that chili powder is not set in stone; you can use fresh chilis (dumped in at the same time as g&g) or chili paste--anything goes.
Besides spices, Indian cooking requires:
-A non-stick vessel. Do not even try to fry spices in a stainless steel pan. It will go to hell--this is a well-attested consequence.
-Some kind of grinding mechanism. Cumin, especially, should be purchased in seed form. One, it's very cheap; two, the fragrance and freshness of just-ground cumin is irreplaceable. As cumin is the leading spice in many curries, it needs to be top-notch. However, seed cumin just as it is has its charms as well. Sometimes you want one or the other, so the best idea is to buy it in seed form and grind as needed. I have a mortar and pestle I was quite intoxicated with for a while, but I'm going to be honest--it's very hard to grind things finely with it without killing the upper arms. I find a grinder (just a regular coffee grinder) saves me a lot of crunchy bits in my cumin powder and energy I'll need to stir.
-Onion and, as aforementioned, garlic and ginger. I find ginger optional and can do just fine without it, mostly because it's a bit of a pain in the ass to peel and grate. Garlic, on the other hand, is really fun because you have to crush it. It adds something indispensible to most curries, and you can never have too much. However, there's still nothing quite like the smell of ginger and garlic frying together. Both is ideal.
-Crushed/diced tomato. I buy it in large tins, one of which will do me for about three curries. Tomato is the ultimate spice-binder and thickener; I have found it very difficult to produce tasty curries without frying the spice-onion mix in tomato first. It takes on a wonderful ripe sweetness as it cooks and prevents a "watery" flavour even if you add lots of water. I find chickpeas especially cannot do without tomato.
-Creaming agent (for lack of a better word). This is the stuff added at the end of the curry to pull everything together and impart a substantial creaminess. You can use coconut milk/cream, sour cream, buttermilk, yoghurt, whipping cream; all of them impart a different flavour to the curry.
-Chopped cilantro. This is not optional. It will absolutely transform a curry. Even the plainest daal will have a spectacular kick to it with cilantro.
-Oil, duh. I put it here because it is extremely important in Indian cooking not to skimp on the oil (or ghee). Everything must be nicely fried, not steamed and scraped at the bottom of the pan. While we're here, the oil vs. ghee debate--
In my experience, they don't taste terribly different, only ghee has a bit more of a pleasant smell to it and gives the frying onions a lovely yellow colour. However, it is more expensive than oil.
-Staples. Basmati rice is the only rice that will do. Bread, however, has a thousand possibilities. Any kind of nice bread will go with curries, Indian or no. Soupy daals especially, I find, are excellent conduits for regular whitey bread.
WHAT CAN BE CURRIED?
Just about everything!
One of the reasons I'm so addicted to Indian cookery is because any vegetable/protein combo works, and patchy fridges don't have to cause any despair. Tofu, legumes, paneer, meat, fish, seafood, leafy greens, beans, peas, carrots, corn (yes! corn!), potato, cauliflower, tomatoes, squashes, peppers, asparagus, okra, eggplant--any one or any number of them together works!
The only guideline is the natural fit of vegetable flavours together. Sometimes the combo seems unlikely but can prove completely addictive. For example, my boyfriend's mother makes very tomato-y chickpea-tofu-spinach curries--absolutely fantastic. There's nothing like them. My own general policy is to put in a legume, then follow up with some frozen veg mix. Foolproof.
While we're on the subject of customizability, one thing is worth mentioning--there's no such thing as an Indian recipe to be followed to a T. How much of this and that goes into a dish is completely up to Individual taste (for example, pretty much the whole world is against me on the amount of cilantro I feel needs to be added to a curry). Measurements are not to be taken as commandments, but loose guidelines; there is infinite room for subtraction, addition and substitution.
Recipes incoming!