Google recently released a new service that lets you perform searches for images that are visually similar to each other:
Google Labs: Similar Images Search
Start by searching for an image using a traditional search term (for example, search for apple). Once you find a result that is close to what you’re looking for, click on the "Similar Images" link and Google finds images it believes to be similar in appearance.
Source: This tip drawn from a mention on the terrific CreativeBits site.