We needed a small cat door so our cat could go in and out of our basement while keeping the (human) door closed so our kids wouldn't fall down the stairs (they are clumsier than the cat).
I looked into pet doors that you install into existing doors, but they seemed a bit over kill for an interior door and none of them would have looked very nice in our woods doors.
We have 6 panel solid wood doors and the thought occurred to me, why not just cut out one of the lower panels and use it as a swinging pet door?
The current version is kind of a beta project just to check if it was possible and that the cat would actually use it. It is possible and the cat does use it. I'll update this instructable or create a new one with any changes. This is my first instructable so I'm not really sure what is appropriate.
First, you should think about if this is the right solution for you.