Below are a few sites I've came across in my past time of being on the web, some already we know about, and some you probably have seen before. However just thought it would be nice to have some sort of list with all of these resources together;
iStockPhoto.com - not free, but great and quite reasonably priced. Flickr.com - just make sure to do an advanced search limiting the results to photos you can use commercially. You'll still have to attribute the author, but not pay (which is certainly acceptable for most corporate blogs or the like).
The Ultimate Resource List
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