On July 4, 1054 A.D., Chinese astronomers noted a "guest star" in the constellation Taurus; This star became about 4 times brighter than Venus in its brightest light, or about mag -6 (whatever that means), and was visible in daylight for 23 days.

Over the fold you can see what it looks like today.

The Crab nebula.

Courtesy of Astronomy Picture of the Day.