My favorite is
morgueFile. I like that you can filter the type that you want ahead of time and don't have to look at the specific license for each picture, after you've decided you want to use it, only to find out it is unusable. They have a very large selection and the photos are good quality.
Here is their license statement for their free photos, "You are allowed to copy, distribute, transmit the work and to adapt the work. Attribution is not required. You are prohibited from using this work in a stand alone manner."