Clarus 1.0 is the latest application of this genre, and it looks like a keeper. The £15.00 GBP program, which appears to have been named after the mythical dogcow of Apple lore, can be downloaded for a free trial. Purchasing the application eliminates built-in limits on the number of contacts, insurance policies, medications, and vet visits.
Unlike other pet management packages, KennettNet built Clarus from the ground up as a Leopard app, and it takes advantage of integration with Address Book, iCal, Quick Look, and AppleScript. Clarus is localized for English, French, German, and Spanish Mac users.
What I find most useful about this program is how it stores all of my pet's care information in one place. Should my cat Ruby require emergency medical care, I have all of her records at my fingertips. If she runs away from home, I can print a "missing" poster with one click.
Clarus gets two paws-up! Moof!