We as an agency struggle with good stock imagery that is inexpensive (as we try to keep our costs to a minimum for our clients). So, you can imagine the joy on our faces when we stumbled across this site – FFCU: http://www.freeforcommercialuse.net/
It offers a broad range of different links to all types of imagery that we will point our clients in the direction of going forward.
P.S. Do remember that images are still licensed – so make sure you comply with them.