The Double Cluster is a pair of open clusters cataloged as NGC 884 and NGC 869, respectively. (They also have the older names of χ Persei [that's the Greek letter chi] and h Persei.) Located in the constellation Perseus, they appear very close to each other on the sky. These two clusters are pretty easisly visible as a faint misty patch close to the border with Andromeda. They are located at distances of 7,600 and 6,800 light-years from Earth, so they are also not too far apart in space.
Anyway, enough exposition, have a picture!
|NGC 884 (left) and NGC 869 (right), the Double Cluster in Perseus.|
Being open clusters, there really isn't that much more I can think to say about them (since they don't have any stand-out features like a planetary nebula in from of them). They were apparently known from at least as early as 130 B.C. (though what they were wasn't know, of course). William Herschel, of Uranus-discovering fame, was apparently the first person to realize that they were two distinct clusters instead of one. Interestingly, they don't have a Messier number, so Messier either never came upon them, or, more likely, found them sufficiently obviously un-comet-like that he felt no need to include them in his list. Considering they can be made out with the naked eye, I'm guessing the latter is more likely.