I always wanted a piece of Star Trek and the Disney Monorail in my house, and one thing they have in common is that they both have automatic sliding doors. It would be the perfect, most geek-ified entryway for my bedroom.
Edit: Not every detail is included in this Instructable. I did the best I could using pictures I had taken 4 years ago. There are some more details on my blog if you would like to read more: http://uiproductions.blogspot.com
To be acceptable as a permanent renovation to our house, I knew the door had to have a normal appearance, as well as be practical and maintenance free. To reduce the number of moving parts (and maybe for a little coolness factor) I decided to make the door air-powered. The air would be supplied by a small compressor and storage tank located in the attic. In order to open and close from the inside and out, the door needed a little bit of brainpower. I decided to use a small PIC microcontroller, my platform of choice still to this day. Arduino didn't exist back then.
With a rough plan in my head, I drew a quick CAD model of the door and the brackets that would connect the pistons to the door halves. I was ready to start purchasing parts.