I recently joined Pinterest, and am completely hooked. If you follow the right things, you can get an overwhelming amount of inspiration and reference. It's also great for an online compilation of illustrators or artists you like. Since building up my boards I've been looking at a lot of different illustrator's work. I have missed hours of scouring the intraweb for artwork and it has led to a lot more sketching. Here is a digital sketch inspired by some concept artists I found.