I haven't really tested any of these freeware weather apps, but there are several you might take for a spin at:
www.freewareppc.com
Because they're scattered through several categories, it's easiest to go to the page above and search for the keyword "weather."
I know you're looking for something that's free, but there are several weather add-ins out there that cost between $10 and $20 that offer exceptional quality and versatility. I personally like the ones that include a variety of skins and/or icons so I can personalize them as much as possible.