While watching extended radar animations is both interesting and entertaining, sometimes you just want to cut to the chase and see how the rain is moving with respect to where you are or where you are going. This is where WeatherSpark's SparkRadar comes in.
The SparkRadar images are computed by analyzing the recent radar images and precisely measuring the distance between the current location and each individual patch of precipitation.
A little patch is then drawn for each such distance and time.
This collapses the three dimensions of [time + two-dimensional radar image] into the two-dimensions of [time + distance] which allows you to read off the most important radar information in one glance.
In a single image this shows you exactly how far away precipitation has been over the last 12 hours. This can be tremendously helpful when trying to assess when a storm front is likely to hit.
This creates very characteristic "<"-shaped patterns for storms that cross over the current location (sloping left as it approaches, crossing the dark red line while covering, and sloping right as it moves on).
For storms that pass closely, but not over, there is usually more of a "rotated-U"-shape to the pattern.
As you look at a couple of SparkRadar images over varying weather conditions, you'll quickly build up a feel for what the surrounding precipitation is doing.
Unfortunately, some locations have persistent noise coming from flaws in the underlying radar signal - doppler radar isn't perfect. Hopefully you will be able to recognize the noise as such and disregard it.
Unfortunately, radar is only available in the US for the time being.