Secret Knock Detecting Door Lock
http://www.instructables.com/id/Secret-Knock-Detecting-Door-Lock/

I like this type of project because it is useful for security purposes. This work as hideout from intruders with a lock that will only open when it hears the secret knock or simply just to open the door when the owner of the place forget the key for example; Tools required for this project seems not to be that expensive since is made with components that we already have and its just the case addition and other small parts that we would have to buy.

Android Soccer Robot
https://www.youtube.com/watch?v=qY4b5sIrGKw

This project is design to play soccer. I like it not only because of the robot movement but because of the fact that it actually holds and throws the ball to the goal net. I guess it has to be include another mechanical component that performs that function.

The Lightning Simulator/Breathalyzer/Graphic Equalizer – Arduino Powered
http://www.instructables.com/id/The-LED-Lightning-SimulatorBreathalyzerGraphic-E/

The LED strips looks great. It is mounted on an outdoor garage roof which functions as a lightning simulator, outdoor breathalyzer, graphic equalizer synced to music, and a few other effects with sound. The color and the effects is the real attraction, use ideally in a dark environment. This project is expensive but good.