Free for commercial use:
www.morguefile.com
www.sxc.hu
www.photolib.noaa.gov
http://pdphoto.org
Other 'free' photo sites - I've never tried them, so read the terms of use first:
www.everystockphoto.com
www.imageafter.com
www.photogen.com
www.freedigitalphotos.net
www.burningwell.org
http://backgroundsarchive.com/
www.unclesamsphotos.com - directory of other US government sites
Google also gave me this list:
http://www.photoshopsupport.com/reso...ck-photos.html
Remember that flickr has creative commons licensed photos, too - the option is under 'Advanced Search' - and that almost all of NASA's images are free to use too.