I just stumbled across a link to the British Library's Flickr page, which has over a million (seemingly) copyright free images and illustrations available for browsing and download.
The topics range from natural history and scientific illustrations to tons of guys with great looking facial hair...
... to historical branding...
... and plenty of great woodcut type and landscapes and, you know, stuff like this:
Lots of cool inspiration and resources for you next project. Someone please make me a t-shirt with that ritual mask on it.
Check it out: Flickr.com/photos/BritishLibrary
Oh, and if you don't know, the U.S. Library of Congress has a huge selection of copyright free art available as well.