One project I am considering building is a homemade remote car starter that would allow me to start my car with my iPhone. This would be much like the Viper Smart Start system. The main reason I would like to build this is to allow my car to heat up on cold winter mornings and cool down on hot summer days before I get into it.
My other project idea, which has been on my radar for quite some time, is replacing the stock radio in my Honda Fit with a custom touchscreen computer. I do love the way the stock radio in my car looks though so I am also considering gutting it and using the interface to control a computer without having a touchscreen interface.