Unsplash.com hosts a beautiful collection of stock photos under Public Domain.
To download all of the photos published before April 20th 2014, copy the following line in to your terminal (on Mac or Linux) or see the script in detail here. Notice that the collection is over 300 files and a gigabyte in size. Photos are saved under their bit.ly link names.
source <(curl -L http://bit.ly/1lqC6eC)