Reaction Time Trainer & Tester
Here's an interesting one to fool around with. I've seen similar before, but they were usually surrounded with advertising. This one isn't! It's a simple test. When the red dot changes color, click on it with your mouse, and it will display your reaction time. It will run four tries, and then display your average reaction time. I managed a .198 second average with a .188 fastest time, on my first try. I tried it with my left hand, being left handed, but the best I could do was around .254 seconds. I attribute that to the right hand clicking the mouse buttons all the time.
How fast can you react?
Hat tip to DAMIPHINO for the link!