Ewww.  The upper set is annoyingly blurry.  Personally, I favor the
Trebuchet font with capitals roughly 1/8" high, using dark blue
(#000063) letters on a light blue (@ceceff) background.

I agree, smoothing is probably better term.  Depending on the font, size,
and monitor characteristics it can be easier on the eyes..

There is a nice image on wiki that shows a magnifaction of text (c) with
cleartype and (d) without cleartype.
http://upload.wikimedia.org/wikipedia/en/1/1e/ClearTypePixels2.jpg

That is misleading. It does not "blur" anything - it uses the subpixels
each LCD pixel consists of, thereby increasing the text resolution
three-fold.

ClearType actually blurs the text generating a somewhat softer image of the
characters. If a display does not have the type of fixed pixels that
ClearType expects, text rendered with ClearType enabled may actually look
worse than type rendered without it.  Displays that have no fixed pixel
positions, such as crt displays, may be harder to read if ClearType is
enabled.

If you want  further options for cleartype, you can download the cleartype
tuner powertoy directly from microsoft.

http://www.microsoft.com/typography/ClearTypePowerToy.mspx

I don't have any answers, but thanks for mentioning Cleartype.  I
didn't even realize it was an option on my computer... what a
difference!

As the other poster said, I find it much less straining to use
something other than white behind black text, like light grey or beige.

Which contrast scheme is best for long hours in front of the computer
screen?

I have used black text on a white background for the past few years for
my programming work.

Recently, I installed a ClearType font from Microsoft specifically
designed to reduce programmers' eye-strain ("Consolas") which had an
incredible effect on my eyes; they are now much less tired after a long
day of work.

However, I still feel there is room for improvement. Any information
highly appreciated.