Windows 7, Firefox, 100 DPI display.
I see the body text on EE.SE pages rendered in Verdana font. I can tell this because of the notorious bug in this font, namely broken kernings in particular sizes. Ironically this is mostly seen in the default size that is used on this site.
It looks horrible because of jumping spacings between letters and this cause quite uncomfortable effect on eyes.
The CSS should be changed to "Arial" or "Segoe UI". I find Arial is better readable in general case, due to less spacings between letters.