The Frinkiac is a search engine and web site that shows screen caps from The Simpsons related to a phrase or word entered. It claims to have over 3 million screen caps.
It's not clear if this site has the legal rights to show these screen caps, but Matt Groening has a history of being lenient on derivative works. FOX and other related copyright owners may not be so lenient.
An example of Frinkiac's image-captioning feature, based on a screenshot from the episode "Homer Badman". The original image can be found here.
Another example showing how the captioning feature can be used to incorporate memes, based on a screenshot from the episode "Marge Gets a Job". The original line was, "Let me be the first to say, kabebu kazini." The original image can be found here.