I have heard of something called 'gruit' which seems to sometimes be used in brewing beer. What is it?
Why are there very few beers on the market containing gruit? What are some of the more popular ones currently?
More than a millenia ago, there were few standardized recipies for herbs to add during the beer brewing process. "Gruit" referred to the herbal mixtures used to flavor and preserve beer. Gruit was usually sold under papal license exclusive to certain areas at various monasteries and therefore represented a monopoly in Christian areas of the Catholic Church. Traditionally it included plants such as yarrow, mugwort, henbane, sweet gale, bog rosemary, juniper, and more. Some of these survive in gin recipes.
Why gruit has largely been replaced by hops is a fascinating and divisive historical question. There may however have been a number of factors including the possibility (largely untested) that hops beers kept longer and the rising concern by Protestant (and even Catholic) nobles of the power of the economic power that this gave Catholic institutions. What is known is that a campaign was waged against gruit and the purported (and likely actual) mind altering properties of it. The use of gruit was more or less entirely supplanted by hops.
More recently some breweries have been experimenting with gruit beers for national pride and/or historical reasons. These include Fraoch and Alba from the Williams Brothers in Scotland, and a few other examples. They are not common however and are certainly specialty beers today. Additionally the gruits used in commercial beers are pretty tame by historical standards. I haven't found any including herbs like henbane (which was one of the historical staples of brewing, used even more prevalently than hops is today).