In a world where every photographer has to worry about their images being used without credit, perhaps the most egregious offense is when one photographer passes off someone else's work as their own. The Tumblr blog Photo Stealers is devoted to calling out the websites that have claimed the work of others, and publicly shaming them into removing it. And wouldn't you know, it's actually working.
Those behind the blog are obviously putting a lot of work into this — they link to multiple images that are stolen, and find the original. At that point, the image owners can contact web site hosts, Facebook, and anywhere else the image is up, and file DMCAs to get the photos taken down. This technique seems to be working, as the blog is frequently updated with what happens on the sites they notice, which usually means the offending images are removed, post haste.
Remember, if someone is using your work without your permission, there's a lot you can do about it, so don't be afraid to go after infringers.