There are lot of things you can use as a "flavoring agent". I've seen people making flavored oils and I made many of those too. Sadly enough they don't keep and by experience, they are always a bit suspicious in my eyes; are they still good or not? So I stopped flavoring oils unless they are used immediately and ristricted my "secret flavoring agents" to making flavored vinegars.

Always use the vinegars sparingly, merely a few drops will do wonders, not only for tweaking the taste, but also for correcting the balance in dishes. It will take many dishes to another level.

Then I tried out other stuff; elderflower blossoms, rose petals, purple basil.

A little different procedure than tarragon vinegar. I cut the blossoms from the flower"umbrella" of the elderflower and put them loosely in a large pharmacists glass jar and cover with vinegar. In another glass jar go the rose petals (I use rosa rugosa picked in the wild), and the leaves of purple basil in another jar. Note; green basis doesn't work well. I leave these blossoms, petals and leaves in their respective jars to macerate for up to two months, then sieve and put in small bottles; look at the color!

Elderflower blossom vinegar; fantastic when used in couscous, fish dishes, when blanching vegetables, a real "waaw" effect in many creamy sauces.

Purple basil vinegar; any preparation with tomato like tomato sauces but also in meat sauces and jus from pork roast etc.

I also tried to make vinegar flavored with common green basil; however, it's a waste of time. Purple basil works best!

Here's a simple dish that undergoes an incredible change in taste, using vinegar and a little sugar. Nowadays you can buy precooked beetroot that's easy to work with. Of course you can boil your own if you like. Peel and cut the cooked beetroot in small cubes. Add red onion slices, a good sprinkle of ajowan (=lovage seeds, ethnic stores) which taste like a softer version of thyme, a pinch of chili flakes, a tbsp of tarragon vinegar or another vinegar mentioned above, a good sprinkle of sugar (1 teaspoon to start), s&p. You can tweak the balance by adding more vinegar and sugar. Beetroot has never been that alive!

Dishes where the sweet/sour balance isn't right will benefit from a few drops of your vinegar, Melissa; tomato sauces, all kind of stews and braised dishes, poaching liquid for fish, in the cooking water of vegetables like cauliflower and carrots etc. It's a matter of being aware of the sweet/sour balance in food. I always taste before serving dishes like that and check for salt, pepper... and acidity. Thing is, just a few drops will lift many dishes to a higher level; flavours will remain much longer in your mouth! Also, in case of adding some vinegar to the cooking water of delicate vegetables like cauliflower, it will keep the cauliflower nicely white.

Raw tomatoes and cooked beet root benefit from just a little vinegar and no oil. Beet root can have quite a bit of vinegar to balance, tomatoes just a little.

And of course, like Brook already mentioned, there's also the vinaigrettes, basically 2-3 parts oil, 1 part vinegar plus your own additions like herbs and seasoning. In one of the first cookbooks written by Gordon Ramsay, I read that he keeps a basic vinaigrette always ready to add a little to sauteed vegetables. For instance; boil green beans, refresh, sweat a chopped shallot in butter, add beans, seasoning and just a few drops of vinaigrette. It's nice to learn that cheffy way of dashing vinegar in a preparation; keep your thumb on the opening of the bottle and let just a few drops at a time pass through. Remember you can always add but never get it out of a preparation.