Jaco was nuts, and it wasn't even the really technical things he did that made him so kick ass...
It was the grooves he'd come up with...
Granted that they themselves were often pretty insane.

More along the Dream Theater line: Symphony X
Twisted, twisted stuff: Buckethead. Try and find Giant Robot or Monsters and Robots. And brace yourself.
Just plain rocks: Clutch - Closest thing you'll find to Classic Rock today. Seek out The Elephant Riders immediately.
Jazz: Charlie Hunter. Plays guitar and bass at once (eight string guitar - three bass strings, five guitar).