Cow magnets are rather large and powerful magnets, and people use them for all sorts of things. But they are manufactured for cows to help prevent hardware disease. That may sound nonsensical, but it's a real condition formally known as bovine traumatic reticulopericarditis. See, cows are liable to hoover up nails and other metal parts when they eat grass, and sharp metal bits can perforate their digestive system and even pierce the heart.
At the age of one year, farmers will give a cow a magnet to ingest. The magnet settles in the rumen, where it attracts any stray metal the cow might ingest. It's an intentionally-introduced bezoar that stays with the cow for life, but keeps small metal objects from messing up the more delicate parts of a cow's anatomy. And now we know why UFOs are able to beam cows up into their spaceships- by the power of magnetism! Read about cow magnets at Stanford Magnets, and read about the fun kids can have playing with these magnets at Metafilter.
(Image credit: Wormcast)