See Readers' Choice Winner for 2011.
About.com Readers' Choice Winner for 2010: Flickr
Photo-sharing site where many of the photographs are available under a Creative Commons license (terms vary). There is a wealth of amateur (and perhaps not-so-amateur) photography talent displayed on Flickr. Users upload their own photos (often in multiple sizes), add tags, and decide what usage to allow. Use Advanced Search to really zero in on what you need.
The other finalists:
- morgueFile: Offers free images with remix, commercial, and without attribution licensing.
- stock.XCHNG (sxc.hu): Find free photos by search or through a variety of categories. Terms vary.
