Price Point Underhelmet Cap
The Price Point underhelmet cap will keep your head warm and dry during those long excursions on the road or trail. This isn't the polyester you remember from 1974, this fabric is moisture-wicking, and one of the most breathable materials around. It is extremely soft to the touch and fits comfortably under a helmet.
I bought this, but it doesn't fit my head (L-XL helmets). It comes down to about an inch above me ears. Problem is, at $5, it's probably not worth the time/hassle to return.