I had been noticing that my older monitor was blurry in a few spots (in two columns down the screen). I realize now that this was a problem with moiré. To see more clearly how it was working, I opened GraphicConverter and created a black-and-white checkerboard pattern. I saw exactly where it was blurry, and I went to adjust the settings on my display. I tried hitting the 'Auto' button and this fixed the problem, even though this had no effect without the image on the screen â€”- I guess this pattern gives a better signal so that the display can calibrate properly.
I've created a 1024x768 checkerboard GIF -- this compresses down to an 8KB ZIP file, so that's how I'm making it available.
[robg adds: Here's a macosxhints mirror of the file, in case the above download link no longer works.]
Mac OS X Hints