Nintendogs + Cats. By using the AR Cards included with the 3DS system, you'll be able to beam your virtual pet into the real world: in your hand, on your desk, or wherever you place the card. With the 3D effect turned on, the simulation of pet ownership must be that much more convincing.
If you're unclear how augmented reality works, you should check out this video of the PSP version of EyePet. Like Nintendogs, EyePet uses augmented reality to add a virtual pet to your surroundings -- sans 3D, of course.