My favorites, in no particular order (some having been previously mentioned):

Petopia's sock puppy trying to get into the kitty's apartment, bribing the doorman - "I'll give you thr - four dollars!"

Petsmart or Petco, where the dog and cat have stacked pet carriers, chairs, goldfish bowls on dog's back so cat can sneak a peak at their hidden Christmas presents.

Advantage - with all the puppies and kittens singing to the tune of that Peter Pan song - "If you don't use Advantage scratch your ear! If you don't use Advantage scratch your rear!...."

Herding Cats!

Lassie saving the world!