Flickr Commons publishing a huge amount of copyright free photos

Take a look at Flickr Commons if you want historic photos with no, or free licenses. Check the remarks on that.

Scene from Fuerth, Germany

You can search for topics and more, and access this huge database. You will find a lot of photos scanned from old documents. Highly important shots, or just random life all around the world. I'm sure if you need to refer to something in the past, this could come handy.