omar rodriguez-lopez from at the drive-in and mars volta
this guy is a genious, he writes so much music I dont think he has time for anything else
when mars volta isnt active, which doesnt seem to happen that much, he goes off and writes jazz fusion stuff of big proportions
kjartan sveinsson from sigur rós - plays a lot and I mean a lot, of different instruments effortlessly, classically trained on many of them, can compose on them all
other good ones I'm familiar with
jimmy lavalle from tristeza and the album leaf
all the amiina girls
all four of them play various instruments like keyboards, guitars, saws, glockenspiels, violins, and other stuff
I could be wrong but I think animal collective has some good ones, arcade fire probably as well
björk
beck
trent reznor
mikael åkerfeldt from opeth
doesn't play that many instruments but he's gifted on them, really gifted
the main two guys behind broken social scene
varg from burzum
got sent to prison, couldn't play guitar, so he, normally a blackmetal man, composed ambient albums using a keyboard
then there's lots more Im forgetting