National Weather Service

Enter your city/​state, and you get a fairly basic weather summary. My favorite feature is the «Detailed Point Forecast». This is such an awesome tool. You can click on a point of the map, and you get a custom forecast for that area (I’m assuming based on the overall forecast, elevation from map, and geographic features).
So, when I went snow shoeing a few months ago, I clicked on the very ridge I was going to hike, and I got a very awesome forecast — and it was dead on! Recently, I went camping on the Lost Coast of California, and I was able to get the weather for patch of beach we were going to pitch tents, and it was totally spot on as well.
This is my new go-​to back-​country tool!