Neon sign transformers are largish transformers that step up mains power to 6-15kV to power neon signs. They also happen to be very good fun to play around with if you are interested in high voltage stuff! Being cheap if you can find them for second hand on an auction site, they are simple to use, and relatively safe (NOTE: they still produce enough voltage/current to be lethal!) compared to other high voltage units due to their built in current limit (to around 30mA).
- Jacob's ladder's.
- A pre-cursor to a voltage multiplier to get insanely high voltages (100kV+).
- General high voltage fun.