Color Test Results is a summary of the results of surveying a bunch of Internet users about what colour text on what background they found the easiest to read. The summary quote:

As you can see, the most readable color combination is black text on white background; overall, there is a stronger preference for any combination containing black.

Not coincidentally, most of my text is black on white (or black on slightly off-white). I was interested to see that white on black actually rates fairly highly, because for me bright colours on black (like white, jwz's green, etc) rapidly give me eye-searing, headache inducing afterimages.

(The survey is pretty old, but human perception is unlikely to have changed that much in ten years or so.)

(From Bill de hÓra.)