A few art/graphics blogs or sites that I visit when I've been staring at the same walls for too long. Great visual breaks:
Lines and Colors for a changing on-line gallery show; Punch and Judy for beautiful scratch board illustrations (and theater design too); fun ink illustrations at Mattias Inks; and 50 Watts for killer book illustrations.
And here's a cool history-of-design site: History of Graphic Design. Look at the history of the poster section.