NWS - Doppler Radar National Mosaic
and There is another I used to use but I can not find it (I like this page better anyway)
This is a radar mosaic built of weather radar responses from around the contential US (ConUS) which allows you to follow storms as they cross the country.
From this page you can also link to other "smaller range" like "Southeast" or "Northwest" (close ups in short)
And from there to various state and specific radar "towers"
A very useful page. They have both looping and static maps, the link is static,