Flickr Commons contains a vast array of free-to-use public domain images from many collections, well tagged and annotated.
In general, I prefer Wikimedia Commons (http://commons.wikimedia.org) though it is slightly harder to find public domain images there (many are CC-licenced) and the consistency is not as good.
Address of the bookmark: http://www.flickr.com/commons
I am a professional learner, employed as a Full Professor and Associate Dean, Learning & Assessment, at Athabasca University, where I research lots of things broadly in the area of learning and technology, and I teach mainly in the School of Computing & Information Systems. I am a proud Canadian, though I was born in the UK. I am married, with two grown-up children, and two growing-up grandchildren. We all live in beautiful Vancouver.