Cumul.us is a place to get simple, accurate, and useful weather information.
What makes it simple and accurate is that it collects weather forecasts from several sources and combines them together to give you a more accurate average, using the idea of the "wisdom of crowds". In short, cumul.us is the "wisdom of clouds". Not only is there data from meteorological sources, but people can make predictions themselves.
What makes it useful is that it gives you the information you really use the weather for every day, like figuring out what you should wear. Using the aggregated weather data (and other people submitting what they are wearing) it will suggest what you should wear.