Verjus (or verjuice) is made from the juice of unripe grapes or. It was the prime source of sourness in the Medieval kitchen, but fell off the culinary map when lemons and tomatoes started being imported. Recently though, verjus has experienced a revival — largely thanks to Australian vigneron Maggie Beer, who produces verjus and has written widely on the subject, as well as developing lots of recipes to showcase the product.
Verjus is not as sharp as vinegar, but is more tart than white wine, making it an interesting and versatile ingredient to cook with. Traditionally, verjus was a cheap artisanal product, and there are lots of guides on how to make your own online. Commercial verjus is tricky to track down in Britain, but is available in a few specialist shops. It's also possible to purchase bottles through online shops of numerous New World vineyards.