8/20/11 UPDATE: Just a few days after this post went live, the developer of MenuWeather Lite announced that MenuWeather Lite would be discontinued. His web site indicates that it will remain in the App Store for a month or two, but I can no longer find it in the App Store. The paid version, now $1,99, is still there.
If you want to be able to quickly check the weather on a Mac, check out MenuWeather Lite. The app places an icon in your menu bar that displays the temperature and general conditions, such as cloudy or sunny, at a predetermined location. Click on the icon, and a dropdown list appears, showing you more detailed current conditions, and a five day forecast. The drop down also contains a link to a 10 day forecast, hour by hour forecast, radar map, and weekend and monthly forecasts.
