Here’s an incomplete compendium of (mostly) public domain pictures.
Images created before 1923 are out of copyright, no matter what their original status was.
Library of Congress Prints & Photographs Collection (http://memory.loc.gov/ammem/index.html) -- There are over 100,000 historical images on this site.
Fromoldbooks.org -- Just what it sounds like -- high-quality scans from old books, all out of copyright.
National Atlas (http://www.nationalatlas.gov/) has all sorts of useful maps, all free of charge.
New York Public Library Digital Image Gallery (http://digitalgallery.nypl.org/nypldigital/index.cfm) -- There are over 500,000 images in this online collection.
GIMP-Savvy (http://gimp-savvy.com/PHOTO-ARCHIVE/index.html) Indexes photos from NASA, NOAA (National Weather Service), and the Fish and Wildlife Service in one convenient search engine.
Web Gallery of Art (http://www.wga.hu/index1.html) -- another good resource for old art, completely kosher because they don't put up pieces from after 1850. The images are high-res.
EveryStockPhoto (http://everystockphoto.com/) This one is good, and not listed on Wikipedia. The site boasts over 380,000 free stock photos, with no usage restrictions (unlike Yotofoto; see above).
Openphoto (http://openphoto.net/) -- CREATIVE COMMONS LICENSED, which means you need to cite the photographer when you use it.
Flickr Creative Commons Pool (http://www.flickr.com/creativecommons/) – Use with caution, these photos are under different kinds of Creative Commons licenses. There are literally tens of millions of images here.
Archive.org (http://www.archive.org/index.php) Not really for images, but great historic video and audio collections, whose copyright status is clearly marked. An excellent resource.
Indymedia (http://www.indymedia.org/en/index.shtml) -- News photos of current events are posted here and at several subsidiary sites (linkable from the main page) under a Creative Commons license, which means post WITH ATTRIBUTION. This is a fantastic resource if you want news photos for your diaries.
MorgueFile (http://www.morguefile.com/) -- An excellent and exhaustive database of surplus stock photos. Use with attribution, otherwise you're violating their copyright.
Dover Clip Art (http://www.doverpublications.com/dspa023/) -- Just what it sounds like: free clip art from the Dover publishing house. You have to sign up for a weekly e-mail digest to use this one, but once you've done that you've got over 400,000 clip art images available for use WITH ATTRIBUTION.
WPClipart (http://www.wpclipart.com/) Clip art optimized for Word documents, but eminently usable online. Unfortunately, the site uses a Google interface, so you have to click through a few pages to actually see the images, but the upside is that you don't need attribution for these.