Just random ideas I think of that use X10. Feel free to add your own.
Alarm Clock:
Use timers and macros to wake you up each day with all your lights flashing and maybe the power horn siren...
Christmas:
Make your lights controllable online using Active Home and modules for each light set.
Maybe make Music Macros that control your lights to the sound of music (similar to my Movie Macro idea lower down)
Halloween:
Dim your lights randomly for a nice effect.
Control Leaf Blowers
Control Smoke Machines
Control Everything really
Movies:
Macro Movies
You have probably seen those "4D" attractions at theme parks where they have extra lights, fans, water, and vibrating seats in addition to a 3D movie. Now I am thinking, why not use X10 to do the same thing in your home with a general set of rules we would all use in order to let us all share macros that control a standardized list of devices. We would use the M house code (for Movie) and have:
M1 = Reserved for theater lighting such as the overhead lights and ground running lights (should not be used in the macros)
M2 = Left Side Strobe Light
M3 = Right Side Strobe Light
M4 = Left Side Lamp
M5 = Right Side Lamp
M6 = Left Side Fan
M7 = Right Side Fan
M8 = Massage Chair thing (I am thinking those ones that you just put on top of an existing chair)
This idea would generally only work for SciFi and Action movies, but that is what we all care about anyways right?
I am going to try to script a macro for a movie and see how it goes, I may have to write a program in PHP to handle all the events, or pay somebody to make a small application that will play the macros with a delay starting out equivalent to the delay between a command being sent and a module turning on. So you would calibrate, then hit play on the movie and your computer at the same time and enjoy the show/experience.
Other Possible things to add to the experience would be a smoke machine with a universal module to control the switch that lets out smoke. Heat Lamps for when there is an explosion in movies. A slightly modified water pik (dental device that sprays water to clean your teeth in place of flossing) to mist water (or I guess you could just have it spray you).
Sports:
When a team scores, set off a police beacon light or any other lights.
Stocks:
Have a red and a green light with a module for each. Control which is on based on how your stocks are generally doing.
Weather:
Have a red and blue light with a module for each. Control the dim of each based on the current temperature. May want to put the lights in a black box to make it easier to see.