Simeis 147. Source
This supernova remnant, the gaseous and dusty remains of a star, is a long way off, but it's really, really big. Even though Simeis 147 is 3,000 light years away it still takes up nearly 3 degrees of the night sky, that's almost 6 full moons worth. That corresponds to Simeis 147 being 150 light years across. Too bad it's not nearly bright enough to be seen with the naked eye.