Marshall Amps...
...and Les Paul guitars. (Actually, it's the headphones that kill you)
I had a bit of tinnitus myself, but in the last few years it seems to have gone away. (What!?)
Mine was a background 60 cycle hum (very slight at it's worst) brought on by sustained exposure to twin Humbucking pickups, and the Marshall set to “11”.
Socrates II's post is a little chilling, when you think about a few that may be mis-diagnosed as schizophrenic.