Skype Car was a little robotic car in a small, boxed off part of my room. Hundreds of people around the world could drive it around via the web and see a video feed of it via Skype, all in real time. Here's the YouTube video I made for it, which has over 18,000 views:
The process to play with Skype Car was long but not very hard. A user would go to a web page and click the start button, which initiated an auto-answered Skype call with my computer. The user would then share their screen, after which they returned to the web page. Clicking the arrow keys on the site would change its background color, which, through the screenshare, was related back to my home computer screen.
There, a color sensor picked it up, translated it into a message, and sent the command to the actual car via BlueTooth, which would then obey the command. For the Google Science Fair, I had different people do this, and tested how well they could complete simple tasks.
After a minute of inactivity, the car would use its ultrasonic distance sensor to navigate its way back to its custom wireless charging station by following the walls. (2019 update: in hindsight, this was… dangerous.)