Free Digital Photos
Another great site, though you're better off going through the menu of categories than searching for a particular image. The organization of photos is the BEST I've seen from most free sites. No registration required. Offers free downloads provided you'll post an attribution license where the downloaded images appears on the web, e.g. Free image courtesy of FreeDigitalPhotos.net". Otherwise, you'll have to purchase a non-exclusive licensed image, starting around $3.
FREE images for personal use on websites, NOT as part of a promotional piece you intend to sell. An easy registration process and good to high resolution quality photos. Read the section 'Availability' for usage terms before you download any image - some photographers offer their work under an attribution license, e.g., "Image courtesy of XYZ.com" while others, such as the one shown above, fall under the standard license.
Getty Images bought Stock Exchange so when you're searching, retrieval includes extremely prominent links to somewhat expensive iStockPhoto images (don't say you weren't warned by the huge iStock watermark). I am NOT a fan of iStock, so it's a little annoying.
This site has a one-step registration process; no email validation required, with an especially helpful FAQ section that explains terms for downloading. The download process is just as simple. Somewhat wearisome - those oh so prominent links to Shutterstock.
Roni Loren's ordeal should serve as a reminder to every blogger never to assume that images on the internet are free to use just because a search of Google Images brings them up. Her post lists great, free sites as well.
Need more resources for stock images? See my earlier post.