According to Wikipedia, Kvass is a fermented beverage made from black or regular rye bread. The colour of the bread used contributes to the color of the resulting drink. It is classified as a non-alcoholic drink by Russian standards, as the alcohol content from fermentation is typically less than 1.2%. Overall, the alcohol content is low (0.05% - 1.0%). It is often flavored with fruits or herbs such as strawberries, raisins or mint. Kvass is also used for preparing a cold summertime soup called okroshka.
This is interesting. I would brew something like this just to try it.