Any decent pet store will have a wide range of collars for cats on offer. These vary from the functional (eg identifying your cat, flea collars) to stylish leather cat collars to the downright over the top.
When my cat was a kitten I attached a bell to his collar. This helped me to know where he was in the house and to find him when he got himself lost or confused.
Whilst getting your cat microchipped is by far the safest and most permanent way to make sure your cat can be identified, adding a tag to its collar with ownership details can also be helpful.
Many cat flaps come with an electronic or magnetic key that attaches to a cat’s collar. This allows the cat flap to make sure it only lets your cat in – not every cat in the nieghborhood!