GMT Slider is a simple site with a neat function. You select different cities or time zones and it adds sliders for each one. All the sliders work together, allowing you to see different times in different places on earth. As you start to type a city, a drop down selection menu appears allowing you to select it.
It is fun to use and is great for teaching about time zones. I was using it to see what time it was for some of the people that I have in my PLN around the world.