Stack Overflow really has a nice clean interface, but I always find some little things I like to tweak - with Stylish and/or Greasemonkey - in the sites I visit most.
So far I have replaced Arial (I really can't stand that font) with Droid Sans and also replaced the fixed width font with Droid Sans Mono. I also switched off the little red
x from the interesting/ignored tag boxes on top right -- I believe they look a lot better without those, and I can remove them from my profile's pref page.
So I'm curious, have you tweaked your interface and how?