This doesn't include buying stuff because he is on their albums e.g. Tweaker.
I certainly have. Some I was aware of but hadn't investigated previously - some were completely new to me.
I started looking into Mark Isham stuff because of his appearancess on the solo albums and now I probably have about 40 of his albums (he's done a lot of Soundtracks in case you didn't know)
Other notable mentions:
Virginia Astley - bought 'Some Small Hope' and then the album 'Hope in a darkened heart' but she captured my heart and I went on to buy everything I could find of hers. If you like pastoral piano stuff you must try 'From Gardens Where We Feel Secure'
Jon Hassell - loved his work on Brilliant Trees and Alchemy so went out and bought a load of his stuff.
Robert Fripp - I go more for the soundscapes than the King Crimson stuff
Trey Gunn - bought his album at the Sylvian & Fripp gig
Bill Nelson
Sakamoto
I'm sure there will be loads of others I've forgotten - remind me fols

Sonic