Speaking of eyeballing, Matthias Wandel took a break from woodworking long enough to create The Eyeballing Game. It's a simple, in-browser test of visual acuity that rates both your eyeballing accuracy and your speed.
Anyone from carpenters to designers to architects—basically anyone who deals with creating lines for a living—ought give it a go. Warning: A subset of you are going to find it extremely addictive and/or become hellbent on improving your score. (I thought I was pretty good, but I'm consistently getting in the 4.- range. I also discovered that I rock at determining a dead-accurate right angle and suck at parallelograms, as I keep subconsciously trying to turn them into planes with accurate two-point perspective.)
Play it here.