How it works: Making a Dashboard widget isn't really that hard now, but it's about to get a lot easier. There'll be a little button in Safari that you can press to take a "clip" of a web page and turn it into a widget on the Dashboard that updates as that page does. Found your local weather forecast somewhere, or a webcam that watches the outside of your house? Clip that section of the page into a web clip, and you've got a simple, custom-made widget so easy Grandma can make one. Here's what it looked like in an old build, and it's probably going to be even easier in Leopard.
Who will use it: If it's as easy as Apple says it is (and all indications say yes), everybody. Anything on the web can be clipped into a widget, so if you can access constantly updated content (say, the top story of TUAW?) then you could use it as a web clip.
You can check out all our 24 Hours of Leopard posts here.