Like Seadogg, I generally use the BBC site. However, as he states, the 'definitive' source for UK weather information is, of course, the Met Office:
http://www.metoffice.gov.uk/
It be scientifically proved (using chaos theory) that it will never be possible to produce really accurate forecasts for more than a few days ahead. But, if you want at least a rough guide for 10 days ahead, this is probably the best site:
http://uk.weather.com/
For overseas locations it's best to use the forecasts provided by the local equivalents of our Met Office. These are brought together, in English, on the website of the World Meteorological Organization:
http://www.worldweather.org
Chris