Tod Maffin mentioned this on his blog sometime ago, and it’s too much fun to not pass on. Weather Bonk is a great mashup of Google Maps and The Weather Channel, not to mention a variety of other great utilities.
I use a combinations of widgets to get the current weather for Vancouver on my desktop, but the Route Weather is a pretty amazing tool for mapping out the forecast of your road trip. I also love the variety of webcams that you can locate with the map. The site seems to have a pretty good pulse on what is available for nearly every major city in North America.
Be careful. You could end up toying around with this site for hours.