ForecastAdvisor:

ForecastAdvisor will show you the accuracy of the major weather forecasters, including Accuweather, AerisWeather, Foreca, the National Weather Service, Open Weathermap, The Weather Channel, Weather Underground, Wetter, World Weather Online, and Weather News. We also provide links to your city’s weather forecast from all the other weather forecasters, so you can compare for yourself.

The overall accuracy percent is computed from the one- to three-day out accuracy percentages for high temperature, low temperature, icon forecast precipitation (both rain and snow), and text forecast precipitation (both rain and snow). Temperature accuracy is the percentage of forecasts within three degrees. Precipitation accuracy is the percentage of correct forecasts. The forecasts are collected in the evening each day.

One knock against the aforelinked new weather app Mercury Weather — it uses OpenWeather for its forecast data, and OpenWeather is meh at best for accuracy. ForecastAdvisor will let you plug in any zip code and give you historical data for forecast accuracy by service. Neat idea.