The daily sketches of Guy Denning. Particularly like the centralisation of the image here. Note: Play around with contrast and how darker colours don't need to be confined to the centre of the page, they can be inverted.
Every single day since November 2010, without fail, Bristol-based artist Guy Denning (previously) posts a daily sketch to his Drawing a Day blog (occasionally mirrored on his Facebook page). It’s well worth following.