I've always felt that a screensaver should be useful and attractive. Clear Day Screensaver for Mac is both. This US$1.99 app lets you view the weather in more than 2.5 million cities via a beautifully rendered globe. Information includes the current weather, three-hour predictions and the forecast for the following day.
Clouds are also accurately displayed, along with the current time in either 12- or 24-hour format. Temperature can be displayed in Fahrenheit or Celsius.
I gave the app a spin on my Mac Pro with a 30-inch screen. Animation was smooth, and the forecasts and current conditions displayed matched up with other information I found from typical sources. I searched for some more obscure, small towns which the app found via ZIP code or name. Clear Day Screensaver supports English, French, German, Italian, Japanese, Russian and Spanish.
Clear Day Screensaver can be accessed from the menu bar and previewed at any time, and the screensaver itself launches in accordance with your settings in System Preferences.
There are several attractive weather apps for the Mac, but few are screensavers, so if that's a feature you'd like, Clear Day Screensaver is worth a look. A point of interest: Apple doesn't sell screensavers in the Mac App Store, so Clear Day is actually an app with its own set of controls.
Clear Day Screensaver requires a Mac running OS X 10.7 or greater.