Putting this list together because a search for "free stock photos" always turns up a load of sites that are mainly or entirely payware/premium, or you only get a couple of free images per month.
Bear in mind that even where credits aren't required, they're usually still appreciated.
Totally 100% free photos
Beautiful, artistic high-res images that are totally free to use. Lots of nature shots and roads, but a few objects and people as well. Perfect for the cover of that novel you always meant to write.
20 million images free for reuse, and you can add your own.
"Absolutely free photos" (and they are) with helpful categories to get you started.
Has a helpful "search by colour" feature.
High quality, artistic images, also accepts submissions.
Free photos but beware the premium pics ads
A wide range of free stock photos and illustrations: if you upload ten of your own photos (may as well do something with all those scenic holiday shots languishing on your hard drive) you won't see any adverts for paid sites.
This has an enormous amount of free images, with the usual premium photos bar.
Big range of totally free photos, also has tabs to premium sites.
Scroll down because the top hits are premium ones.
Huge amount of free images between premium bars.
Free photos that require attribution
Flickr: Creative Commons
These images do have various licenses but are free to use in many circumstances.
500px.com: Creative Commons
Huge range of creative commons pics with various licence levels.
Click Search Tools -> Usage Rights -> Labeled for reuse (or the usage of your choice), as Google is an Aggregator the exact rights of these images may vary.
Wide range of photos, some require credits.
Also has website templates, good range of artistic images.