Les Claypool (20+ times) including incarnations of his other bands:
primus
oysterhead
sausage
colonel claypools bucket of bernie brains
flying frog brigade
and i also attended a book reading of his.. does that count?

AC/DC
KoRn (4 times)
Metallica (5 times)
presidents of the united states of america
tool
rage against the machine
soundgarden
neil young
pearl jam
b.b. king
devo
black sabbath (ozzfest)
live
helmet
and quite a few more.. may edit to add later