Try this list...
morgueFile
PixelPerfect Digital
OpenPhoto
Image*After
PDPhoto (PD is short for Public Domain)
FreeStockPhotos.com
Geek Philosopher Stock Photos
Photo Rogue "One of the main goals of the site is to increase the amount of visual media on the Internet that is not incumbered by copyrights." Essentially Photo Rogue is the idea which kicked off the thread. Populated by volunteer photographers, you can even request a shot.
Many of these sites have terms and conditions, be sure to make yourself aware of them. For example, some require a link back to the site you got the photo from. Others want attribution for the photographer. This is not much of a restriction for commercial use. Just about all prohibit one business use: Bundling them and redistributing/reselling the photo collection for others to use, for example a CD of clip photography.
Hint: The Digital Camera is one of the hottest, fastest selling and consequently wide-ranging products on planet Earth. If you can't get the client to point a picture at one of their real, live employees, at least request they smash their cameras with a hammer and mail you the pieces.