Finding art reference, whether it be in the form of a concept art book, a how-to book, a website or the artists of antiquity, is incredibly important to keep your craft up to stuff and keep you always learning. After a friend of mine inquired about which references I find most useful, I thought I would write about it here:
Art books of movies are great resources too, some of my favorites include The Art of Brave, The Art and Making of Cloudy with a Chance of Meatballs, and the art of any Miyazaki movie. Art history books are also great, and often glanced over. My favorite painters are J.M.W. Turner, James Abbott McNeill Whistler, Henri Matisse, Gustav Klimt, Norman Rockwell, and Jacques-Louis David.
-Feng Zhu Design: Feng Zhu is an accomplished entertainment designer. His website is chock full of gorgeous art, and he offers a lot of video tutorials for free that he makes to teach people his process.
-CGSociety: A good resource for looking at art, talking to other artists on forums and keeping up to date on CG news. Although the culture of the website has gotten a bit exclusive in the last few years, I still check back every once and a while. Their work in progress forums are particularly helpful.
-Mayang Textures: I use this website all the time for finding high-quality photo textures for free.
Tumblr is a dangerous beast. Inside a chaotic mix of teenagers being melodramatic, and many wonderful pictures of animals, you can find a great artist community. If you tailor the people you follow to only the best artists, you can log on each day to find a feed full of fantastic art and inspiration. That being said, here are links to some of my favorite artists on there:
I hope these references prove useful! Feel free to comment with your favorite resources.