Anyone who's read through my blog is aware of two facts about me, I'm a tightwad when it comes to buying tools, and I firmly believe in measuring by eye. Refining the ability to measure by eye is actually less about seeing and more about questioning. For instance,when determining whether to surfaces are square, if I look at it and try to make it square, I will quickly convince myself that it is square, or worse, square enough! But if I ask myself, how is it not square? I am on the road to accuracy. It takes a bit of energy, and honesty, but the assumption that you are off will reward you with keener vision and results. My lovely bride sent me this link the other day
This site is fantastic. It is a series of visual tests where you determine by eye certain geometric tasks, and are graded on your accuracy. I have only done it once, and plan to do it again (my score was 2.90). It was interesting using my "shop brain" while at the computer. Take a couple minutes and visit the site, it's a must for shop monkeys!