The Gnomon Workshop offers professional training to any artist in any field. They stock a wide variety of books and dvd's on almost every artistic subject.
Concept Art is a fantastic community or very talented traditional and digital artists. The focus is primarly on 2d work, but even if you are a 3d artist you should definatly check this site out.
Deviant art has some great resources available. There are lots of tutorials, reference images, brushes and models that people can download. Just be sure to check the terms of use on these assets before you use them in a commercial production.