UpperWeather Radar
Radar is my favorite weather tool
Because I love radar so much, it makes me sad when people just want the most smoothed over radar images. The news channels brag about having doppler but I have never seen one show a velocity image.Oh well. I guess it is what you are comfortable with, but keep in mind that each site does not have their own radar just different ways of displaying the data that national doppler radars put out.
Storm Prediction Center
This is the radar site I go to when I want the broad overview (above). It has good enough resolution to see any major events. They also overlay warnings and convective outlooks. SPC has a lot of other great tools check them out.
Intellicast
I think this is the most popular radar for pilots and most people to get a quick national radar shot. Just realize that it is very smoothed over.
UCAR radar
I am most comfortable with the UCAR site. So, this is my favorite radar site. It has almost all the available radar products, and it works fast.
Plymouth State
Plymouth State offers almost every available radar product there is, and definitly more than any other web site except maybe weathertap. This is probobly the best place place to get radar pictures without paying or having a security clearance.
WeatherTap
WeatherTap is probably the best radar site there is but you have to pay for it.
Weather Underground
Weather Underground is awesome on so many levels. The radar stuff is a little smoothed over and they don't offer all radar products but they have an awesome "frequently asked questions" section. They also offer a lot of other support products, and they offer more every time I look at their page, so it is definitely worth a look. The only problem is all the ads which they will turn off for a fee.