I have seen (and heard) Tesla coils used as an audio speaker, but I wouldn't call that a practical use.
I guess you could use one to test electronic components for their resistance to electrostatic discharge or for scaled lightning damage experiments
certain chemical reactions need to be done in vacuum, You can use a tesla coil to determine if the apparatus has been evacuated by allowing the coil to discharge onto the glassware. If you get a glow of anykind within the glass there's still gas in the system. [this is how neon tubes work]
Create high voltage.
Read:
http://en.wikipedia.org/wiki/Tesla_coils
