Use solid-state relays to blink the lights on your Christmas tree in time with music. Also good for Halloween haunted houses
Step 2: Add lights
String lots of lights onto the tree. Use as many sets as you like, but arrange them such that there are three light regions, each with a separate plug. Run these three plugs down the trunk so they can easily be attached to the light controller. Up to three strings of lights can usually be linked in series, so you can almost certainly cover even big trees.
Step 4: Run with it
Fire up the music, and treat your friends, family, and entire neighborhood to a great light now. If you really want to share, hook the music up to your outside speaker system and make the house shake in time to the music.