Yoshinoya is a popular fast-food chain famous for its gyudon, thin slices of beef cooked with onions and a sweet soy sauce are ladled over a bowl of rice. A branch of Yoshinoya is in New York City on 42nd Street. The first shop in the chain dates back to 1899 and was located near Nihonbashi. It moved here to Tsukiji with the move of the market.
Chuo-ku, Tsukiji 5-2-1 Building #1
Phone: 03-5550-8504
www.yoshinoya.com/shop/tsukiji/index.html (Japanese)
Food Sake Tokyo 
I posted a delicious recipe for gyudon:
http://michaelbeyer.wordpress.com/2010/11/13/gyudon-beef-bowl/
Give it a try sometime!