I was exploring a new supermarket this week and spotted a new ingredient to try. I love vinegars and, aside from super-old balsamico from Modena, they are generally a very economical and fast way to add different flavors to foods.
Ume vinegar is made from the brine left over when plums (ume in Japanese) are pickled into umeboshi (pickled plums). As a result, it has a great deep fruity flavor. It's also hella salty (think pickle brine) so you should use it sparingly (Ask poor Beppo about the tablespoon he shot down last night. Well, ask him after he stops coughing, which should be any time now.).
So far, I have used it on a cole slaw (I diluted it with a little rice vinegar to keep things acidic enough without the salad getting super salty) and sprinkled it over some rice (for a sushi rice vibe). I have read that it's good for salad dressings, but I think you'd want to dilute it with more red wine vinegar (again, it's very salty). I think it would be fabulous on a potato salad (no mayonnaise, but with sesame, basil and mint maybe?).
Look for ume vinegar in the Asian foods section of your supermarket, health food stores or Asian markets. My 10 ounce bottle cost about four dollars.