Another of the great free museum of Tokyo, the Sumo Museum is right at the heart of the Sumo world, housed inside the Ryogoku Kokugikan  (the sumo stadium itself). Dedicated to displaying a wide range of materials from 3,700 wood-block prints to embroidered belts, you can learn everything there is to know about Sumo right here. They have the Banzuke: the official rankings of sumo wrestlers today as well as over 500 dolls of wrestlers. The museum is also home to a research center, studying the role of Sumo in Japanese culture.