I love archives, especially archival images. I enjoy touching up old plates, maps, and diagrams found in old books, university libraries, and municipal archives to use as art or give away as gifts. The Public Domain Image Archive aggregates images from more than 200 source institutions and seems like a great complement to classic sources such as the Internet Archive and Wikimedia Commons.
The site is also very straightforward about the limitations of its claims regarding public domain status:
For each image, we relay to the best of our knowledge the rights status of both the underlying work (detailing the relevant region/copyright term) and the digital copy of this work (detailing if any attribution is required). We provide this information based on a basic knowledge of copyright law and the status communicated by the source institution — it is strictly meant as a guideline and it should not be taken as legal advice.
Hat tip to davidbarker on Hacker News.