Forecast.io is a sweet new online forecast website from the makers of the Dark Skies iOS app.
Rather than cram these things into Dark Sky, we decided to do something grander: create our own full-featured weather service from scratch, complete with 7-day forecasts that cover the whole world, beautiful weather visualizations, and a time machine for exploring the weather in the past and far future. You can access it from all of your devices, whether it be your laptop, iPhone, Android phone, or tablet.
Geolocation through the browser, a time machine feature (the day I was born was a balmy 40F), worldwide support, and lots more.
Update - also works on mobile and will let you pin to your home screen. Very impressive.