Okay, THIS might be the geekiest project ever. Some guy has built a kitty security door on his home that uses facial recognition software to identify, and admit, his cat Flo, while locking out all other animal undesirables.
The first issue of MAKE had a much easier, more lo-tek, approach. A crafty pet owner built a little touchplate platform inside the kitty door which was rigged with a big capacitor and buzzer. If a cat lingered on the platform (as a reluctantly entereing non-resident cat would), it would discharge the capacitor and set off the alarm. The home owner, apparently permamently fixed to his easy chair nearby in the den, could then blast the intruding feline with a squirt gun.