Tales From Weirdland has a collection of posts that feature concept drawings from several Studio Ghibli movies like Spirited Away, Howl's Moving Castle, and Castle in the Sky. I poked around a little and found artwork & concept drawings from Princess Mononoke, The Wind Rises, and Porco Rosso. Hand dawn and it all just pops off the screen. Wonderful.
Looks like a lot of this is available in book form as well: The Art of Spirited Away, The Art of Princess Mononoke, The Art of Howl's Moving Castle, etc.