There are a ton of old M42 screw-mount lenses out there for cheap. The catch is that you need an adapter to mount them on modern cameras. One option for Nikon shooters is the Viltrox adapter for Nikon F mount. SCROLL DOWN FOR UPDATE!
The Viltrox adapter does what it claims to do: allow for the mounting of M42 screw-mount lenses onto any camera with a Nikon F mount. The overall build quality is quite nice and there are some well-thought-out features, such as plenty of indents and grooves so you can get enough leverage to remove it from a lens easily.
The advertised standout feature, however, is the glass optical element that, according to the manufacturer, allows the lens to focus to infinity. Due to the thickness of the Nikon F mount, the added thickness of the adapter moves the lens far enough from the film plane to be able to focus to infinity. For a wide-angle lens, this is a major problem. For telephoto lenses, this may not be a big deal (e.g. – for portraiture).
And the Vitrox delivers on that promise. You can indeed focus to infinity with M42 mount lenses on a Nikon body. I tested with a Helios M44 55mm f/2 (my review) and a Rikenon 135mm f/2.8. Both lenses mounted, focused, and otherwise operated as they should on a Nikon D3300 (my review). On the Rikenon, I could actually focus beyond infinity.
UPDATE LATE 2018:
I find that the Vitrox adapter contributes extra chromatic aberration (CA) to lenses that are already prone to such behavior. But, CA is a common trait of older manual focuses lenses in the M42 mount anyway, so if you are bothered by chromatic aberration, I would suggest skipping old lenses entirely. Or, simply just process your images in black and white and totally avoid any type of chromatic problems, be it the green and purple fringing of aberration or the very ugly chroma noise that makes up part of ISO noise.
Beyond the bump to CA, the Viltrox adapter does what it claims to do. And the price makes it a nice value. The world of cheap M42 lenses can be fun to explore, and the Viltrox for Nikon F mount will get you a ticket the party.