In the U.S. I used to use wunderground, until they were bought out by weather.com and put so many gadgets on the page.
weather.gov is now my favorite site for long- and short-term forecasts. Most of the other sites, such as the weather channel and accuweather, take the weather service forecast and tweak it a little bit. The only weakness is that they don't pull from the various TV station radars, so when the local government radar is down (typically for maintenance, but it broke in advance of a line of severe storms earlier this year), I have to play "go fish" with other sites.