In proteins, crosslinks are important in generating mechanically stable structures such as hair and wool, skin, and cartilage. Disulfide bonds are common crosslinks.  Isopeptide bond formation is another type of protein crosslink.
The process of applying a permanent wave to hair involves the breaking and reformation disulfide bonds. Typically a mercaptan such as ammonium thioglycolate is used for the breaking. Following this, the hair is curled and then "neutralized". The neutralizer is typically an acidic solution of hydrogen peroxide, which causes new disulfide bonds to form, thus permanently fixing the hair into its new configuration.
Compromised collagen in the cornea, a condition known as keratoconus, can be treated with clinical crosslinking.
In biological context crosslinking could play a role in atherosclerosis through advanced glycation end-products (AGEs), which have been implicated to induce crosslinking of collagen, which may lead to vascular stiffening.