I want to make a dimming alarm clock. I would like to be able to use it with any lamp I want, so I want to have an AC outlet, but I need away to dim the AC outlet. You can buy cheap dimmer cords that have a slidey switch inline. I could buy one of those cords, or a rotary dimmer, and interface a servo motor to it or something to mechanically move it. But is there a more elegant way to dim 120VAC using a microcontroller?
One approach would be to plug the light into an X-10 dimmer module, then control the module from the Arduino. A little expensive, but... safe.
And once you've mastered controlling THAT X-10 module, a whole bunch of other things become easy.
See Playground for X-10 ideas...
http://www.arduino.cc/playground/Main/InterfacingWithHardware#wireless
(Scroll down a bit, or use browser's "Find in this page")
Of course, with governments around the world writing laws taking incandescant light bulbs away from us in the not too distant future, this may be a project with limited life-span. Do they make dimmable compact florescents? If so, my bad.
OR you could go for bright LEDs... seems that's one of the "options" we are allowed by Them. And LEDs have two virtues: You aren't dealing with the household voltages... dangerous... and they are very easy to dim by pulsing them with different duty cycles.
I'm actually thinking of using a high powered LED. Only a single LED would be bright enough for a night-light, and I could just use the PWM of the arduino into a transistor or LM317. It would still be better to have a universal outlet, though.
Here in TX we have plenty of incandescent bulbs available still.