Free Pictures for Commercial Use.
Flickr: Probably the most popular site for free pictures, but watch out- many of their pictures must be licensed (paid for, or negotiated in some way in-writing with the photographer) to use commercially. However, you can use their search-filter to only show results which use the CCL (Creative Commons License), which means you can use those pictures commercially, including on your website (but you must give a link and/or mention of the photographer). Here is a link to Flickr CCL search to find free pictures for commercial use.
US Government: Any picture (or anything at all, including sound, video, web design) found on a US Government website is automatically in the public domain, with very few exceptions. Of course, this means you can use it, even commercially, with no attribution. Here is a good starting point to find US Government public domain pictures.
MorgueFile: Probably my favorite site for free pictures. Many of their photographers offer the Creative Commons Attribution License, but some don’t even require an attribute. Personally, if I can see who the photographer or author is, I give attribution anyway. But they also have some cheap, paid, high-quality photos too. The quality of their picures is exceptional- the best in the business. Most images are huge, and crystal clear- very professional. Check MorgueFile here for free pictures.
Wikimedia Commons: Another outstanding source for not only free pictures, but video and sounds as well. Most do not require attribution. Some are public domain, some are CCL, and some GFDL (free, even for profit, with some limitations). Read each one before you use it to be sure. Search Wikimedia Commons here.
I will add more later. If you know of any websites that offer free pictures for commercial use, tell us.