Messier 35, off the left foot of Castor in the constellation Gemini, is one of my personal favorite winter sky targets for a small (4 inch refractor) scope. For an open cluster, M35 is quite massive, being comprised of somewhere between 2 and 5 hundred stars (although I cannot see anything near that number). Located at approximately 2,800 light years from our Solar System and having a diameter of something between 11 and 24 light years (it's often a matter of opinion as to exactly where an open cluster stops and neighboring stars begin), M35 has an apparent magnitude of 5.3. So it ought to be visible naked eye under a dark sky (something I myself have never had the opportunity to test). But under the moderately light polluted skies of Howard County, MD, it still manages to look spectacular through the eyepiece, and makes a great subject for astrophotography (or, so I am told). The cluster is just over 100 million years old.
Looking through a scope of moderate aperture, M35 presents a curious donut shape, with an apparently starless hole at its center (see above image). This is an illusion, due to the stars in the "hole" being slightly fainter than those surrounding them. Larger scopes (say, 12 inches) will fill this hole in nicely (see image below).
But once you've enjoyed the view of M35, don't stop there! Because right next to it is the magnitude 8.4 open cluster NGC 2158. Now, unless you've got a really big scope, the most you're going to see of this line of sight companion to M35 will be a faint smudge. So don't expect to resolve any individual stars. In my scope, it looks like a really dim globular. But it is, in fact, another open cluster, this one far more distant than the much brighter M35 - about 16,000 light years away, or well over 5 times the distance to M35. NGC 2158 contains more than a thousand stars and is well over a billion years old. So despite their apparent neighborliness, the two objects are quite different - this is no Double Cluster!
A beautiful image of NGC 2158, off to the lower right of M35