The Genka chain of sake-specialty izakayas is known for their policy of selling sake "at cost." Each branch serves several dozen premium sakes at remarkably low prices, starting at just Y280 per serving. The sake comes in cute 100ml mini-bottles that are the perfect size for sharing, and at these prices it's easy to put together your own tasting flights, a convenient way to sample several different types of sake over the course of an evening.
Genka's food menu changes regularly, and although it takes a back seat to the sake, we've found some very good dishes here. Note that there's a cover charge (they call it a "member's fee") of Y500 per person, which easily pays for itself by the time you've ordered just a few servings of sake.