If you've ever had a stray cat come in through your pet door at night, you know the chaos thhat can ensue. To eliminate that issue, you can construct a secure pet door that only lets your cat or dog in after scanning the RFID tag on their collar. For added excitement, the system also takes a webcam photo of your pet entering or exiting and tweets the event.
This project started as an Instructable from user e1ioan, but has now grown so popular that e1ioan has created a site for the project and offers kits for those who need a little extra help.
It isn't a cheap project -- you'll need a RFID reader, a micro-server to act as a latch control, a webcam, a router running Open WRT, and several semiconductors, capacitors, resistors and other electrical hardware. The source link below offers loads of schematics, step-by-step instructions, and the complete code needed for the project. If you're a techy pet-lover this might be for you.