I've had a DeviantArt account since 2009, but haven't done much with it until recently. I've been using it lately as it has a lot of stock photography and some of it is useful in a pinch. Also you can find PhotoShop brushes and vector shapes and a variety of other handy art references. In addition it provides interesting data when you post art; you can see who has favorited the art and what else they've favorited to give you some insight into who likes what you're posting.
It has become a place where I post drawing experiments. Some experiments include these mixed-media drawings I've been doing for fun with pen & inks coloring them with watercolors, colored pencil, and finishing them digitally. Here are some examples.