What kind of photos are you looking for? For photographs (not icons or drawings), I like to use Flickr. Basically, you can use anything there listed under the Creative Commons license that's matched to your particular situation. Just click on "advanced search" at Flickr, then check the box that says "only search within Creative Commons-licensed content."

If you’re not familiar with Creative Commons, basically it’s a way for artists, photographers, writers, etc. to license their work so that other people can use it under certain conditions (usually attribution is required). CC is a great way to gain exposure for your work. That’s where I source most of the photos for my blog, and I really appreciate all the amazing things people share under CC.

Today I discovered http://www.freepixels.com. They have a decent selection and you don't need to register.

I've used stock xchange for many years... http://www.sxc.hu

I use commons.wikimedia.org they have creative commons, GPL, copyright-free, and public domain photos. I prefer public domain stuff.

If you use WordPress you can even use a great plugin called PhotoDropper to find creativecommons photos from Flickr.

Tagaroo, another WordPress plugin, will also search for Creative Commons-licensed photos based on the tags that you assign to a post. If you write directly into the WordPress editor, Tagaroo substantially slows the process of writing a post, because it also suggests tags as it parses your content. If PhotoDropper avoids that problem, I think I'd opt for that one. On the other hand, if you're like me and you compose posts in an outside editor and then cut/paste into the WordPress editor, Tagaroo is pretty great.

One really cool resource for honest-to-goodness, free stock photography is Free Range Stock.