I love sauces. Besides white sauce, I don't make many sauces from recipes. Instead, I just mix a little of this, little of that until I have something I like. Usually I survey what is in the fridge and cabinet, and figure out what sort of flavor I am in the mood for, like, do I want something rich, tart, salty, sweet?
Ingredients to keep around:
Liquids: wine, vermouth (very useful), chicken stock, vinegar, olive oil, sesame oil, soy sauce, fruit juice, worchestershire, lemon or lime juice
herbs and spices: garlic, onion, ginger, black pepper, cumin, fresh herbs like parsley, rosemary, basil... I could og on forever
other flavorful stuff: mushrooms, greek oil-cured olives, pimentos, roasted red peppers, cranberry sauce
One of my favorite mixes is vermouth, roasted red peppers, garlic and olive oil... really yummy over chicken and broccoli.