From the NASA Image Galleries:
On April 25, 2008, NASA’s Swift satellite picked up a record-setting flare from a star known as EV Lacertae. This flare was thousands of times more powerful than the greatest observed solar flare. But because EV Lacertae is much farther from Earth than the sun, the flare did not appear as bright as a solar flare. Still, it was the brightest flare ever seen from a star other than the sun.
Image Credit: Casey Reed/NASA