The same day that GazoPa's similar image search graduates from beta, Google upgrades its own Google Labs similar search experiment to a full-fledged Google Images feature. A "Find similar images" link under certain pictures pulls off the trick.
From a cursory few searches, Google's own similar image search seems to perform a bit a bit more accurately when it comes to hunting down real-life images that look like other captured images. You can't upload or draw your image basis, as with GazoPa, but then you get greater results filtering with Google as well — exact dimensions, colour saturation, file type and more.
Both sites are worth keeping bookmarked if you're often digging around for something just a little different than what you're working with.