Pixactly is both a game and a quiz that tests your knowledge of graphic space in pixels. You are challenged to draw a box of specified pixel size, both horizontally and vertically. The game keeps track of how many pixels you are off over five attempts.
You’d think I would be good at this after nine years of blogging, but no. My score the first time through was 584. Maybe I can blame that on the fact that I zoom in and out for different websites, depending on the text size. So I tried again and cheated by taking a screenshot, which displays pixel size, before guessing. I got my score down to 136 that way, which is still worse than any graphic artist. You will do better! -via Metafilter
Have you tried this?
http://woodgears.ca/eyeball/