A light organ is a piece of equipment, often used by disc jockeys at parties, to flash colored lights with the beat of the music. After adjusting the sensitivity, when the thunder booms, the lights will flash. Some people like to use strobe lights for the lightning. I have been told that flood lights work better. Strobe lights tend to be fickle, requiring all sorts of electronics to get them to flash intermittently.
Flood lights are generally cheaper and easier to work with.
You can buy the floodlight sockets with an attached stake for outdoor use. Simply stake them in your yard. Or hang them from the eaves of the house, or up in a tree. Set up the floodlights to flash onto the front of your house or across your Halloween display. Of course, we all know that you often see lightning before you hear the thunder.