Weather you're a data lover, or a flexible planner, I love using NOAA's hourly weather graph feature.
Go to www.noaa.gov, type in your zip code (or city and state), and then scroll down and look on the right for "Hourly Weather Graph." It's great for trying to plan when the winds will be lowest or the temperatures highest for squeezing in that mid-day bike ride or a morning ski tour.
Bonus points go to the clickable point forecasts, back on the 7-Day forecast page, which allow you to get weather predictions for remote, pinpoint areas such as mountain tops or your favorite backcountry ski run.
Weatherspark is a cool website which has combined this hourly data with historical averages, on one plot. It's nice, but the historical data is usually drawn from the nearest airport, which can be really inaccurate in the mountains.
For realtime weather in your area, go to weather undergrounds wundermap, which combines measured temperature and windspeeds with a local radar on an easy to use interface!
For non data-heads, I also find goingtorain.com entertaining.
Update: For snowy weather, check out my post about snow forecast tools