You can't beat Flickr. There are millions of photos available for free under Creative Commons licenses, here:
http://www.flickr.com/creativecommons/

The images aren't (always) professional quality, but a little cropping and  color/contrast adjustment can do wonders.

I forgot to mention...

Beth Kanter does a great job using Flickr photos on her blog, here:
http://beth.typepad.com/beths_blog/

I like flickrCC at http://flickrcc.bluemountains.net/

I've recently launched a new site for free photos,

Ookaboo:  Free Pictures of Everything on Earth - http://ookaboo.com/

although it just started in July,  Ookaboo has already gathered nearly 700,000 pictures of 380,000 different things.

Unlike some "free" photo sites,  all pictures on Ookaboo are public domain or creative commons and can be used freely for both commercial and non-commercial uses -- so it's maximally useful to non-profit and other organizations.

Hi Paul--

This is really cool, thanks for sharing about Ookaboo. I was very interested that you all are using a semantic web API. Would love to hear more about you & your background to learn more about how this site came about.

Best,

Megan

Quite good source of free photos is Open Image Bank (www.openimagebank.com). It is very simple, the images are available for any use - commercial and non commercial and there is no need to register.

To limit your search on Flickr.com to those photos that have Creative Commons status that allow for non-commercial (and sometimes commercial) use, you should first click on  "Advanced Search" , then scroll down and click the box for Search Only Creative Commons.

I like two free photo websites:

Jeyjoo Images - www.jeyjoo.com/gallery - good for macro and travel shots - all images are free

RGB stock - www.rgbstock.com - more images but registration required!

My favorite source for free stock photos is this site www.kozzi.com. I've been using it for quite a long time now and I really like it. And downloading photos and images in this site is totally free. :)

"And downloading photos and images in this site is totally free. :)"

Not entirely... "Subscribe today and download up to 30 images per month for free or upgrade to a full monthly membership for $4.95USD per month." They also push an affiliate program.

Also, I'm not sure how the site uses the information of those that sign up in order to use the site for free - the privacy policy won't download for me.

Taking this in a new direction - why use stock photos at all? Nonprofits, NGOs, schools, libraries and other mission-based organizations work with volunteers, clients and others that would be happy to have their images used in promotional material!

Sometimes I like using icons as visuals as opposed to photos. I heavily use iconfinder.com for that purpose. If you sort by "Allowed for commercial use (no link required)" you can ensure copyright compliance.