Suki suggested Poster Street as a Pick of Day.
This is what they say about themselves.
"We developed Poster-Street.com to provide you with motivational, funny
and thought provoking wall art that makes a difference. After scouring
the web we realized there was a need for high-quality posters that
were well-designed and easily accessible. Feel free to use these
wherever you like and make sure and tell all your friends."
At Poster-Street you can download free posters as PDFs. Here are some examples: