A while ago I hooked up a camera to monitor my cat door.
I was curious to know how may cats beside my own two cats were entering the house. The gear I use are a Raspberry Pi, the noIR camera and two reed relais that are switched by the cat door. Lighting is done with a bunch of IR LEDs. The Pi uploads the images it takes to a Flickr page.
There is also a page to view the latest or random images taken by the camera.