Apprently people who've been doing creative editing on Wikipedia didn't realize that when you hit the "save" button, the old version of the document, including your IP address and the original text gets saved.
Wikiscanner lets you go back in time and see comparatively what's been changed in an article. It also lets you search by IP range, so if you know the IP address range of some pharasudical company (which isn't hard to find) you can quickly see what changes they've made to wikipedia.
Needless to say, this tool has been used to expose some amazing acts of whitewashing by people / corporations / government agencies / etc.
What's that old saying "sunlight is the best disinfectant."