You've got a cat, friends who also have cats, and an arrangement. The arrangement is that your friends cat sit while you're away and you return the favor when they're out of town. So is it necessary to offer a gift or money as a thank you?
When the favor of cat or pet sitting is being bounced back and forth and is a mutual agreement, there probably isn't a need to show appreciation with a gift; however, there are always caveats.
For instance, one listener had a friend bake a cake to show appreciation. So bringing a small gift for that person might be in order next time s/he takes care of your pet. Perhaps someone watched your cat for an extended period of time; that could be worthy of a gift since, otherwise, you would have paid for a kennel.
But be sure not to raise the reciprocity. Keep the gift-giving to a minimum or, better yet, just maintain the in-kind agreement. Hear more on the topic by listening to this episode of Awesome Etiquette.