Here's a gadget you can't buy, but you can build. It paid off twice for Brian, first by solving a household problem, then by entertaining us all on the 'net!
The problem: his cat would get up on the kitchen counter and destroy the houseplants. He wasn't just eating them; he would play with them and drag leaves through the house. I have a similar problem in that one of my cats believes it is beneath his dignity to drink from a pan on the floor, no matter how fresh the water is. He would much rather drink directly from the faucet or from dishes in the sink. The solution: scare the cat. Brian rigged up a motion sensor connected to a loud blender and a strobe light. An overview of the process is included on the Blender Defender page. The best part is the security camera trained on the area to capture the action. When a motion sensor episode is detected, his computer assembles the images into a flash file, then notifies him wherever he is so he can watch the latest feline humiliation. Warning: cat lovers may feel guilty about laughing at this, but the feeling will pass.
See more video clips at the Blender Defender site. There's also a price list for the items if you don't already have them laying around, which doesn't include the computer setup. Without the camera, you could rig up this training aid for not much money at all. But the video is priceless!