There have been a number of unusual items counted as wills. Most notably in Canada, the fender of a tractor was once probated.
Recently, in Sydney, Australia, a suicide note was considered to be a valid will. The note stated that all of his assets were to be left to his mother, rather than to his estranged wife. Otherwise, he would have been found to have died without a will and his wife would have been the sole beneficiary.
While the mother and wife in this case were able to come to an agreement, it is important to be sure you have a proper will in place so that no one needs to go to court to determine whether you did.