A gamma ray burst that occurred 7.5 billion years ago was visible on Earth by the naked eye this week. It was "2.5 million times brighter than the most powerful supernova ever seen."
Here's a History Channel video about Gamma ray bursts. "Scientists at the University of Kansas believe gamma ray bursts were responsible for a great mass extinction on Earth 450 million years ago. The gamma rays strip away the ozone layer and generates a chemical smog, producing a widespread chill that grips the globe. Every few seconds, a supernova emits jets of deadly gamma rays somewhere in the galaxy. If one of these gamma ray bursts should happen sufficiently close to the solar system, all life would perish."
Research has been conducted to investigate the consequences of Earth being hit by a beam of gamma rays from a nearby (about 500 light years) gamma ray burst. This is motivated by the efforts to explain mass extinctions on Earth and estimate the probability of extraterrestrial life. A gamma ray burst at 6000 light years would result in mass extinction; a 1000 light year distant burst would be equivalent to a 100,000 megaton nuclear explosion. A burst 100 light years away would blow away the atmosphere, create tidal waves, and start to melt the surface of the earth. There is a one in a million chance that there could be a gamma ray burst as near as the earth's closest star, Alpha Centauri, in the lifetime of the earth. Such a burst, at 4.3 lightyears distant, would effectively incinerate the earth.