Knitted Head Warmers made by MEEE!
These are great for days that aren't quite cold enough for a thick hat but definitely not warm enough for no hat at all!
They keep the wind out of your ears, keep them warm and still allow circulation around the head when you need it.
I've even sometimes added one of these over the top of a hat when I want a bit of extra warmth and protection for my ears.
 
One size: they're pretty stretchy so will fit most heads large or small.
10cm wide