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Most organisms are normally diploid, but polyploidy may occur due to abnormal cell division. It is especially common among ferns and flowering plants. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. In some situations polyploid crops are preferred because they are sterile. For example, many seedless fruit varieties are seedless as a result of polyploidy. Such crops are propagated using asexual techniques such as grafting.

Triploid crops: banana, apple, ginger, watermelon, citrus
Tetraploid crops: durum or macaroni wheat, maize, cotton, potato, cabbage, leek, tobacco, peanut, kinnow, Pelargonium
Hexaploid crops: chrysanthemum, bread wheat, triticale, oat
Octaploid crops: strawberry, dahlia, pansies, sugar cane

Apples are commonly found as both diploid and as triploid.

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Excerpted from: Polyploidy