This project by Greg is about building a smart litter box for his cats:

We have three cats and the litter is difficult to control. So I had been thinking through a project to build an enclosure. Once I stumbled on Arduino the doors of opportunity were opened. I ordered the electronics and got started on my project right away. So far the Arduino is activating exhaust fans, lightng, and a Lysol spray dispenser. It tracks the number of times the fans are activated and uses a piezo buzzer to alert a filter cleaning. I plan to use it to trigger cleaning based on usage and track each cats potties so we can control their stink before its too late.

A nice video about this project can be found on YouTube.