There are TONS of weather sites on the Web, but these are some of my favorites

Weather Data

Oklahoma Weather Roundup
Real Time Weather Data from NCAR/RAP
RAMSDIS Online - a great satellite page
Unisys
Ohio State Weather Server
COLA/IGES
Michigan's WeatherNet
The Storm Machine
HubCAPS
Interactive Weather Information Network
Storm Prediction Center
Current Watches and Warnings from Texas A&M

Weather Organizations

Center for Analysis and Prediction of Storms
Oklahoma Climate Survey
National Severe Storms Lab
National Weather Service
National Climatic Data Center
American Meteorological Society
Tornado Project Online

Want to know what this all means?

Online Guide to Meteorology