I tend to use Stock Xchange: http://www.sxc.hu/ Most require attribution to the photographer, but that's fair enough.
One of the best places is Flickr. You can do a advanced search based on the type of creative commons license you want. You should find hundreds of thousands of photos. Some will be free use with credit and others will require you to contact the owner and ask permission. Best place to go in my opinion. Most are photographers who would love to have their work spotlighted and get some recognition (photo credit).