A nifty little Flash game
The Eyeballing Game
-- the object is to move a point around on screen to estimate the location of the center of circles, the fourth corner of a parallelogram, the midpoint of a line segment, and so forth. Good practice, as well as a fun challenge.
(I got 2.22 on my first try, and 2.42 on the second one when I was going a bit more quickly. Oddly enough, subsequent tries are slowly but monotonically getting worse; I guess patience is important!)