I can't believe I didn't highlight this utility when it first came out, but the recently updated Pocket Radar is a clever idea and implementation. Given that you know how far something is travelling (e.g. around a track or across a marked distance), Pocket Radar automates the timing of the traversal and does the speed calculations for you. Screens, etc. below.
Here's Pocket Radar in action:
Very helpful instructions come built in, in this case solving my question of how to change the distance units between imperial and metric. Let's keep this simple, measuring a few Usain Bolt 100m races from the TV, though in your case it would probably be a local runner or cyclist or driver... or whatever else you wanted to measure the speed of....
The main UI is elegance itself. You simply tap and hold the main 'Measure' button for as long as the action is happening and the speed is calculated and displayed instantly, the moment you release your finger. (Right) even better, a log is kept of previous measurements and you can toggle betwwen the main Pocket Radar speed display and the log view at any time, perhaps to see how successive runs/attempts/whatever have worked out.
Like most Pico Brothers applications, the user interface is well crafted and easy to understand and everything just... works. I can see many people with an active or sporting lifestyle making great use of this. You can buy Pocket Radar for £1 here in the Nokia Store.