good list of free photo sites

For another in my series of "good places to find free or royalty-free images online for your library stuff," check out "10 Places to Find Free Images Online and Make Your Content More Linkable," by Loren Baker in Search Engine Journal.  There were three sites I hadn't used before, but found to be quite useful and content-rich.  Creating an exhaustive list of good sites in this category is pretty much impossible, but this top ten list is definitely a good place to start!