Verjus, the acidic sour juice of green grapes, was extremely popular in sauces and dishes in the Middle Ages, but now replaced in most recipes with wine or vinegar.
You can buy it in small bottles in gourmet shops (it costs more than many wines). French restaurants sometimes offer chicken au verjus, and some chefs use it in a salad dressing as it’s more compatible with wine than vinegar or lemon.
Images by BorgQueen at English Wikipedia., Dana Facaros