His work with Crimson is brillant, not that Fripp can't handle all the guitars himself but having someone like Belew certainly helps.
Aside from Crimson, he has appeared on a handful of classic albums, this includes Talking Head's Remain In Light, Paul Simon's Graceland, Frank Zappa's Sheik Yerbouti and Nine Inch Nails masterpieces The Downward Spiral and The Fragile.
He has also contributed to albums from David Bowie, Peter Wolf, Tori Amos, Porcupine Tree, The B-52s, Bela Fleck & The Fleckstones, Peter Gabriel, Joe Cocker, Cyndi Lauper, David Byrne, Tony Levin, Herbie Hancock, Mike Oldfield, Robert Palmer, Tom Tom Club, Crash Test Dummies and William Shatner.
That's right folks, William Shatner.