OK, actually nothing's just as good as truffle oil. Bad example. But there are a lot of ingredients that you can substitute without harming the outcome of your dish.
Often, you can pretty easily replace a more ancillary ingredient you'd have to buy with something you already have at home. For instance, in many recipes, you can use the following:
- Dried herbs instead of fresh
- Vanilla extract instead of vanilla beans
- A cheaper cut of meat for a more expensive one
- Your own homemade infused oils (like basil oil) instead of store-bought
- Swiss cheese instead of Gruyere
- A mixture of onion and garlic instead of shallots
- Yogurt instead of buttermilk
- Vinegar instead of lemon juice
To find out what would make a good substitute for an ingredient you don't have at home, just do an Internet search for "substitute ______." Keep two things in mind here, though: First, always Google the amounts when you substitute, because they may not be the same for the original and the substitute items; and only do major substitutions for ingredients that aren't the central element of the dish.