Fun little game from Ze Frank that I hadn't seen before: Every Second Counts. You're challenged to hold the mouse button down for 0.2 seconds, 0.4 seconds, then 0.6, 0.8, and so on. You need to be within 0.1 seconds of the target time to advance to the next time. Because the increments get increasingly smaller in comparison to the overall times, it quickly becomes difficult to gauge how long to hold the button, i.e. 0.4 is twice as long as 0.2 but 3.2 and 3.4 are almost indistinguishable. (It's also difficult because the button is kinda hinky.) I made it to 1.8 seconds...is it even possible to get to 4 or 5 seconds?
I found this via Frank's recent post about differences in scale.
Update: Several readers made it to 4, 5, and even 8 seconds. Most were musicians who have strong sense of timing. I'm also reminded of a story about how Richard Feynman developed his sense of timing to the point where he could keep time in his head even while reading. (thx, everyone)