Gyūdon (beef bowl) is a bowl of rice covered with beef and onion, with a sweet soy based sauce. If you prefer it with pork instead of beef (like me...), at Yoshinoya you can order butadon and at Sukiya you'll find tondon.
There are over 1000 Sukiya restaurants in Japan and in Tokyo you can find them in almost every neighborhood. My favorite was the one I found on a quiet backstreet from Shibuya, in Sakuragaoka-cho. It was a cold winter evening, I was frozen and the appetizing smell coming from the restaurant urged me to go inside...
EXIF info:
Nikon D90
Lens: VR 18-55mm F/3.5-5.6G
Focal Length: 18mm
Aperture: F/5.6
Shutter Speed: 1/100s
ISO Sensitivity: ISO 200