Google recently launched a "reverse image search" service that allows users to search the web for similar images. Simply drag an image file into the search box and it will find everywhere on the web that your image appears.
So, I used the reverse image search last week to look up my own work and to no surprise, found more than a few companies using logos I designed on their websites. There were even a few designers who took my logos, changed them slightly and then claimed them as their own work on their websites.
Try it out > > > Reverse image search.