A huge range of websites seem to offer weather information, though until now there has been a fairly limited range of integration with desktop apps (other than through Dashboard). The idea of integrating a weather forecast with iCal is one which seems obvious, but has only recently been introduced in the form of WeatherCal.
WeatherCal is a $10 System Preferences app which adds a five day weather forecast into iCal for cities of your choice. Forecasts appear as all day events, and are easy to sync with your iPhone or iPod Touch. This review will provide an overview of WeatherCal as well as a couple of solutions which provide very similar functionality for free.