The New York Times website has an incredible series of satellite photos showing the effects of the Japanese earthquake and tsunami. They use a slider effect to allow you to instantly compare "before and after" images of the same scene, and it's a very effective technique.
I was curious about how they accomplished this. I assumed it was done in jQuery, and I was right. A quick search uncovered this tutorial and I couldn't wait to try it out.
I don't know whether I'll ever have a practical use for this technique in my work, but it's a cool effect.