Here is just a good all around source of web links courtesy of Jeppesen CFI refresher course. Thanks to those guys over there and the AOPA Air Safety Institute!
The National Weather Service (NWS) site has official forecast products including graphic weather products, SIGMETs, and NOTAMs.
NWS’s Aviation Weather Center is becoming an increasingly important source of preflight weather information for pilots. Through this FAA-sanctioned site, you can obtain reports, forecasts, charts, including nearly real-time NEXRAD radar, and all weather information for a standard briefing.
Intellicast offers a number of graphic weather products, including advanced radar data from various sites.
Unisys provides an archive of graphical weather products on its weather site, which are helpful in identifying trends and historical weather patterns.
The Weather Channel is one location to obtain recent radar data for a particular region, as well as general forecast information.
NOAA posts a site with information from wind profiling systems across the nation.
The Aviation Digital Data Service (ADDS), sponsored by the National Weather Service, provides graphics depicting winds, turbulence, PIREPs, and other aviation weather data.
The National Weather Association is a good site to obtain educational materials relevant to weather.
AOPA gives access to certain Jeppesen weather services for its members.
Jeppesen provides weather services to pilots and flight departments on a fee-for-service basis
Source: Jeppesen CFI Renewal Online-Weather for Pilots