MOBIUS - Not a ton of stuff in the web pages, but if I was in Boston I'd visit the place
Digital Gallery - Poorly organized, semi-ugly web gallery on a slow link from Brazil, but if you look long enough, you will find some wheat among the chaff in the images
Woltjen Art - Canadian Art Dealer, if more "traditional" contemporary art is your bag, you might dig some of the pieces on display
Mindbreaker Studios - Some nice semi-erotic CG images on a poorly organized web site
Abulafia Gallery - Fairly cool personal gallery. A nice VRML version of the gallery is there, but he doesn't really understand the difference between SGI rendering and PC rendering (small nit)
STM Image Gallery - Scientific Visualization art, some nice stuff. When, however, are people going to get past puttting these weak maps of their galleries laid out like a floor plan? Get with it! This is a new medium! Explore it!