GRLevel3 -
GIS Line file mapping (Street level).
Pan and zoom.
GPS aware.
Customizable.
Can look at more than on radar site by running multiple apps.
Does it's own rendering making smaller downloads.
If the computer supports it, you can "smooth" the radar data (I don't, but some people love it).
Easy screen captures in PNG format.
Level III radar information is free (NOAA public Servers aren't always reliable though. This is "fixable" using data feeds from other sources).
Warning Polygons are displayed on screen and are users on/off selectable along with TVS, Shear, and Hail markers
METAR information on screen.
Vertical Wind Profiles available
Can view up to 4 radar products in a single window.
I can't say too much about Weather Tap. I've only seen it a few times. It appears that it loads Gif files from the internet. It's browser based. Zooming is there, but appears only to be zooming on the previously rendered image.