I'm not normally a fan of Windows screen savers. They don't really save anything at all, and merely take up space on my PC. But I have to admit to rather liking YoWindow (www.yowindow.com).
Not only does it give you a 3-day weather forecast, and show various figures about current temperature etc, it also includes a simulated landscape picture of the real-time conditions. As evening falls, for example, the image gets progressively darker. And the amount of sunlight in the image depends on the actual temperature in the city that you've configured the program to display.
If you don't like the landscape image, you can even create one of your own, based on a photo. There are lots of helpful hints on the company's web forum, which you can reach via www.yowindow.com.
YoWindow is free, for a version that provides the 3-day forecast and up to 2 cities. For unlimited cities and a 6-9 day forecast, it costs $10. It runs on all versions of Windows from XP upwards, and is a 4 MB download. Advisory: YoWindow Free is bundled with OpenCandy. More information here.