Not sure how much of a secret this is, but the closest thing I can think of is oregano flakes. In tomato spaghetti sauce, beef stews… just about anything savory and wet that’ll be cooking for a long time. 
Ground Ginger in most things I make. Steak, stirfry veggies, chicken, it doesn’t matter, a dash of ground ginger along with whatever other spices I feel like that day.
Finely Chopped Crystalized Ginger in my scrambled eggs. (usually with fresh diced mushrooms and onions cooked right in.)
2 drops of Tabasco Sauce per egg in my omelets. Nobody has ever known there was hot sauce in them (even those who say they hate hot sauce) but everybody raves about how good they are.
Worcestershire sauce.
Rooster Brand hot chili sauce. It’s Asian but goes well with non-Asian dishes. I find the flavor fuller than tabasco.
MsRobyn will appreciate this: St-Hubert BBQ (rotisserie) chicken seasoning. A combination of salt, “spices” (I’m sure there’s pepper in there), onion and garlic powders, and (yes, I know…) MSG. Kind of like seasoned salt. I season flour with it when dredging meat before searing.
Also, a lot of regular (not sweet) paprika in dredging flour gives veal, chicken, or turkey scallopini a nice golden color.
Celery seed.
Sour cream in brownies makes them even richer and more chocolate-y.
Extra-sharp cheddar cheese mashed into potatoes is damned good, too.
Old Bay Seasoning. Good:
on Deviled eggs
for touching up boring soup.
on french fries
for making just about anything peppy but not too hot
Seasoned rice wine vinegar, in a lot of things.
What was he doing in the cabinet to begin with? Trying to hide him from ACBG?
More OT: whole wheat flour. In any kind of savory bread I make, I always substitute at least part of the flour with wheat flour.
Actual vanilla extract rather than the “artificial vanilla flavoring” nonsense. My mom smuggles it in for me from Mexico.
Pepper in french toast.
Maple syrup in fruit pies (not a lot, just a little).
Love
…what?
Old Bay (you MAD’s know what I’m talking about), and cumin. The cumin is what makes my grilled meats to die for. Burgers, hot dogs, steaks, chicken, fish, whatever. With cumin in it it’s all good.
A decent dollop of Soy Suace in the butter-sauteed mushrooms I put all over a nice grilled steak.
A technique rather than an ingredient: Whenever I use dried herbs and spices, I always pulverize them with a mortar and pestle before adding them to the food. It releases much more of their flavor. I use a Coors ceramic mortar and pestle I purchased from a science supply house for the purpose.
It does wonders for any kind of potato anything. Potato salad? You bet. Mashed? Oh yeah. Home fries? Mmm-hmmm. Scalloped? You have to ask? Pour it on!
Sazón by Goya. In everything. Tuna salad, scrambled eggs, hamburger patties, chicken, spaghetti sauce, chilli, deviled eggs, soups…
It really is all-purpose. It will put a funny yellow tint to your food if you use the one w/saffron.
Oh definitely. Go mortar and pestle! (Or a spice grinder works well.) Anybody in the Chicago area who wants to get a huge ceramic mortar and pestle for the low, low price of $20 should go over to the Asian grocery store on the southwest corner of the first street east of the Red Line on Argyle Street.
I don’t know how much of a secret it is, but I love fennel seed when paired with pork. My favorite loin dish is simply pork encrusted with cracked fennel seeds, pepper, and salt. I throw fennel into all my pork rubs.
I also like a bit of fennel seed in my tomato sauces.
For mashed potatoes and chicken soup, I will often throw in some freshly chopped lovage. Nobody ever knows what in the hell it is, as you pretty much have to grow it yourself if you want it.
Well, it’s not “cooking” for people, but I’ve also noticed that crushing catnip in a mortar-and-pestle makes my cat go bananas. It’s a “quantum leap” over the reaction he displays when I just dump out some sprinkles from the bag.
pulverizing herbs = gooood.
A dash of ground cinnamon on my chili. Mmmmmm
Cinnamon in chocolate. Not enough so it tastes cinnamony, but adds lovely depth to the flavour.
Balsamic vinegar- on grilled veggies or added to bolognese, chilli con carne or any meat stew.
Tabasco- I add a drop or two to most tomato based dishes
Lea and Perrin’s Worcestershire Sauce- a drop or two in soups, stews, bolognese, chilli con carne and anything with beef or lamb in it.
Cinnamon on porridge (oatmeal)
I don’t add salt to anything (heart smart and all that), but I add extra Black pepper instead.
I make Thai style stir-frys from scratch. I use a stick of lemon grass, some scallions, a 1" bit of grated root ginger and a clove of garlic to flavour the (sesame) oil, then add a big bunch of coriander and a tablespoon of soy sauce just before serving. Just add chicken and veggies.
Lemon pepper - it goes on damn near everything except desert.