Bike Helmet Earmuffs
Removable earmuffs that attach via velco to your bike's helmet.
I like to bike around in cooler weather, but my ears can get very cold in the breeze. At the same time, if I wear a stocking cap or winter hat under my helmet, I can overheat and my helmet can become too tight and uncomfortable. I also have a tendency to set down hats and whatnot in random spots, so I don't want to worry about keeping track of a specific headband or buff.
A few years ago I made some earmuffs that can attach easily to a bike helmet - it attaches with velcro to the underside of a helmet to keep your ears out of the cold but also allows the top of your head to vent if you work up a sweat. It's a semi-permanent solution that'll be able to stay on your helmet for the entire cold season, but can be taken off and stored away in just a few seconds if it's not needed for your ride.
The inside is a soft flannel, and the outside is ripstop nylon. It attaches to the helmet via velcro and to the chinstrap with an elastic loop. Instructions on how to place it can be found here.
These are $20 for a pair (good for one helmet). Colors can be cobalt blue, charcoal gray, burnt orange - can provide others as a special request, as well.