Animal Planet: Saying Goodbye: How to face the biggest decision of all, when to end your cat's life.
Care for My Cat: Feline Euthanasia and Coping with Cat Loss: Explains what euthanasia is and how to cope with the death of your cat.
CatStuff: Helping Your Cat Deal with the Loss of a Companion: Provides information on how to help your pet cope with the death of a companion animal.
The Feline Advisory Bureau: Euthanasia: Describes what happens when a cat is "put to sleep". Also grieving and how to help children to cope.
Vetinfo: Euthanasia: Mike Richards DVM explains what happens when a cat is euthanized and whether or not to consider euthanasia for cats with feline leukemia.