When you light a scene with multiple lights, it can be tricky to see which light is affecting which area. To turn a light off, select the light in the Hierarchy panel. Then, uncheck the checkbox at the top of the Inspector panel. Poof! The light is gone, but not forgotten. Check the checkbox again to make it reappear.

In fact, you can turn any GameObject on and off by using this checkbox. It's a handy way to isolate things in your Scene without deleting your hard work.

If you're working on a particularly grim game and you want to be able to see what's going on in your Scene, toggle the internal lighting by pressing the sunny little button at the top of your Scene view. When it's lit, your light Game Objects take over. When it's unlit, internal lighting hits your meshes so that you can see what's what.