WeatherCal is a system preference pane that displays up-to-date weather conditions and forecasts in iCal, using data from the Weather Underground Web site.
Once installed, you just open WeatherCal in your system preferences, type in the city or ZIP code for as many areas as you'd like to follow, and then current conditions and multiday forecasts (updated hourly) will appear as all-day events in iCal, within individual calendars. You can also click on each event to see a link to the Weather Underground Web site for more information, and you can sync calendars with your iPhone or iPod Touch.
Unfortunately, much of this functionality can be obtained for free from the Weather Underground Web site, just by clicking on the iCal link on any conditions and forecast page (or by copying the URL and subscribing to the calendar directly through iCal for continued updating).
WeatherCal does offer some nice extras, including weather icons, a streamlined process for adding and removing cities, and over-the-air syncing to the iPhone and iPod Touch--but we hope to see more features in future updates to justify its relatively high price.