Nikon Speed Booster?

I've heard of the Metabones speed booster, which allows full frame lenses to be used on APC-S bodies. I was wondering if there's one which allows a Nikon FF lens to be used on a Nikon crop body. E.g. 50mm f1.4 ff lens -> 35mm f1/0 equiv. Are there any alternatives to the metabones speedbooster?

No such. Need not too.
Nikon FF lens can be mounted directly on Nikon crop body.

Both FF and crop bodies have the equal lens-to-sensor distant (technically called Flange Focal Distant). There's no space left to put extra glass in the between.

The same goes on to Canon lens/body pair too.

Focal reducer is possible only for lens system that has more FFD than the body system.

You can use full-frame and film lenses on APS-C bodies without employing any hack. Focal length and aperture sizes won't change though. A 50mm f/1.4 FX lens will still be a 50mm f/1.4 lens on an APS-C body. Angle of view will become narrower (50mm x 1.5) at 75mm (35mm equivalent) but maximum aperture size will remain at f/1.4.