I am forever grateful to the people who contribute images to Wikimedia Commons and make them available under CC0 licenses. It's a tremendous resource for open-source projects, where typically you don't have the funding to pay for the rights to use a stock image from one of the commercial services.
Today I wanted an image of a baler (a farm machine that makes bales of hay) for use in a new software project, and found a nice one by Glendon Kuhns.