NOTE: These are all links to external sites.
There are 2 Direct User Access Terminal (DUAT) providers. These organizations are funded by the FAA to provide weather briefings to pilots.
DTC's DUAT servce can be accessed at http://www.duat.com.
Dyncorp's DUAT service can be accessed at http://www.duats.com,
Some of the FAA's Automated Flight Service Stations (AFSS) have their own Web sites, although typically these don't provide weather briefings they provide other useful information on obtaining and using weather briefings. Find these sites at http://www.faa.gov/offices/index.cfm#AFSS.
Weather reports and forecasts are handled by the National Weather Service (NWS) at their site http://www.nws.noaa.gov/. Aviation weather can be found at http://aviationweather.noaa.gov/.
One of the NWS's useful pages contains an outline of a Standard Weather Briefing allowing you to pick through the various products to review the weather http://aviationweather.noaa.gov/awc/aviation_weather_center.html.
Finally this is a Government source, but it's covered with disclaimers, however the Aviation Digital Data Service does have some great products http://adds.aviationweather.noaa.gov/.
The Airline Dispatchers Federation has a great set of links to briefing information at http://www.dispatcher.org/brief/adfbrief.html.
The Unisys corporation makes a number of weather products available at http://weather.unisys.com/.
If you're an AOPA member then weather information is available from them at http://www.aopa.org/members/wx/.
A company called Intellicast provides a number of weather products at http://www.intellicast.com.
Some of the best real-time radar images can be found on the Web sites of your local TV station. In the Raleigh-Durham area I use:
If you have suggestions for other aviation weather related Web sites please send them to me at email@example.com.