Artemisia vulgaris Mugwort - As the name implies, it was traditionally used in mugs of beer before the popularity of hops.
Prior to the 1600's in Europe, most beer was spiced with an herbal mixture called Gruit. The exact blend of herbs and spices was a closely guarded secret, but commonly used were mugwort, yarrow, cinnamon, nutmeg, and nettles. Mugwort has a mellow sage-like aroma, but it has strong bittering properties and is one of the best herbs aside from hops for this purpose.
From 1/2 to 1 oz. of dried Mugwort for bittering is sufficient for a 5 gallon batch.