Modern 

FredArt
largely anime -- and the driving force behind Megatokyo

DeviantArt
open forum for art, photography, skins, etc.  (pg-13)

  Exploding Dog
moody & minimalist.  and...umm...sometimes a little weird.

Traditional

CGFA
broad database, good depth on each artist.
use the mirrors if this particular link seems slow.

The Art Renewal Center
enormous, but only representational art (portrait/landscape, etc).  often the best resource for the artists it covers.

Mark Harden's Artchive
similar to CGFA, slightly less friendly interface.