Accuweather.Com
Local five day weather forecast, current local weather conditions, radar, satellite images, weather maps, and temperature band maps for over 43,000 locations.

CNN Weather
Top stories, weather maps, storm center, business traveler, and four day forecasts for 7,200 cities worldwide.

Intellicast
Clickable U.S. image displays greater detailed weather predictions. Also has selected major cities by region.

MSNBC Weather
The latest top weather news from the television-internet industry leader.

National Weather Service
Why go anywhere else when you can get the information straight "from the horse's mouth," as the saying goes. Outside companies often get their information from the NWS.

The Weather Channel
Just like the television channel -- all weather, all the time. Enter a city or U.S. zip code for local forecasts.

USA Today
Weather forecasts & news. Also see the special section on Weather Basics.

Weather Underground
Specializing in real-time delivery of weather conditions, forecasts and warnings in more than 35 languages worldwide.

Weatherbase.com
Weather records and averages for 12,222 cities worldwide.

Historical Weather Data
You can find the exact conditions for most cities around the world at Weather Underground. First, find the weather of interest using the box labeled "Find the Weather for any City, State or Zip Code, or Country." Once your selected weather forecast appears, scroll down the page to "Weather History for This Location." Select the month, day and year of interest (note: only last 5 years available) and hit the "Go" button. The weather history includes average, maximum and minimum temperatures, heating degree days, dew point, precipitation, snow depth, sea level pressure, standard pressure, visibility, wind speed, gust speed, moon phase and any unusual events that may have occurred on that day.
There is also a search available at the National Climatic Data Center