Stock Images

So I do quite a bit of freelance web design for some pocket cash. My issue always comes down to content. So a lot of times since the client can't produce images and text quickly enough I like to insert some stock images. Does anyone here know of a decent site to find some free stock images that I can use for the sites?

I do not intend to sell these images. They will simply be used as placeholders until the real content is brought in. Please keep in mind that I WILL NOT just grab any old image from any site. I will only use royalty free (legal) images.

wikipedia is great for that sort of thing. many of the images on it are public domain, you'll just have to click on it before hand to make sure.
also: http://en.wikipedia.org/wiki/Wikipedia:Public_domain_image_resources

Make them yourself?

I usually just make a big white square with the dimension written on it.