[pvaldes]

Normally to other plants, rarely for animals (see: poison ivy, poison oak and poison sumac). In really extreme cases can be very dangerous even to burn it (but nobody has a manchineel in their garden). Walnut can poison the soil for a couple of years so is better put its fallen leaves on a isolated compost pile. It depends on the species and the chemistry of the soil.

[yesenadam]

Wow, thank you, I had no idea. The walnut tree uses the poison to kill competitors, up to 20m away or more.

Black Walnut: The Killer Tree https://www.extension.iastate.edu/news/2005/jul/070701.htm

https://en.wikipedia.org/wiki/Juglone

"Juglone is occasionally used as a herbicide. Traditionally, ...has been used as a natural dye for clothing and fabrics...and as ink. ...has also found use as a coloring agent for foods and...hair dyes. ...is currently being studied for its anticancer properties"

[pvaldes]

And is not only the walnut, a lot of common trees do this in fact. Pines for example.