Since this site seems to have mention of all other types of Automatic Cat devices. I figured I should mention the Automatic Cat door that I am using.
Power Pet Fully Automatic Pet Door, made by High Tech Pet. http://www.hitecpet.com/powerpetdoors.html
It has to be the most expensive pet door I have purchased, but it is very well constructed and has features to support the price tag. It has an active collar that you put on your cat, and then the door actually opens itself when your cat get near the door. It also forms a very nice windproof seal, so no cold draft in the winter I've been using it for over a year now, and have had no problems with the door.
I needed a pet door that would prevent racoons from getting into our house, which as we were to discover takes quite a bit of doing. Most of the other "locking" cat door are designed in such a way that the racoon can either just force the door open, or pull the door open and slip inside. This door slides vertically, so there is no opportunity for the racoon to either push, or pull the door to make it open.
The only downside is the collar they supply, isn't a safety collar, and the module is larger than I would like, but it is easy enough to switch things around to a safety collar, and the size doesn't bother the cats.
Order an extra collar, since the cats have lost one, and I have had to return one for RMA, and it's nice to be able to either test or replace it if you need to.

