If you're like many people, the first place you ever encountered fundoshi was through the Taiko drumming group Kodo, from Sado Island off the coast of Japan. Not only is Kodo a renowned international drumming group, they are also members of an artistic commune of about 48 members who rigorously train for the primal, thunderous Odaiko (literally "big drum") solos that are highlights of their concerts. Their discipline is admirable, honing the body and mind into a rhythmic force that should really be seen -- and felt -- to be believed.
There are several very nice books of photography of Kodo, including DADAN by photographer Takashi Okamoto (published September 2009) and KODO by photographer Komaro Hoshino (1984), which focus on the sheer raw muscular power of the drummers, catching the beads of sweat, the fierce facial contortions, and bodies sharpened by the vigorous, spartan lifestyle of Sado Island.
There are also several other fantastic Taiko groups operating worldwide. Of special note is Kodo's predecessors, Ondekoza.