These were exactly what I was looking for:
-they DON'T fall off your feet because of the adhesive portion in the heel
-they DON'T show on even my most open flats
-they're comfortable and light enough that my feet can breathe (and don't sweat) in them
-my toes don't feel cramped when I wear them

I wear an 8 or 7.5 and got the M-L size which fits perfectly.