Mountain Hardwear Transition Dome - 2010 BCS

Keeps my head nice and warm. Amazing for how thin it is, which is awesome because it fits under my climbing helmet for winter climbing. Windstopper works great for keeping you warm, and dry also because it sheds snow pretty well. The fleece keeps your ears toasty, and is good and soft too so no chafing against the ears. The large size fits my pretty good size head well, and isn't too tight.

The cap is warm for being so thin. It's also water resistant enough for snow. It also has a soft lining around the bottom so it feels good on your ears. However, it's tight. I have a medium head and bought a regular according to sizing chart. I should have gotten large. YMMV.

Warm and fits nicely in my pocket! Not too tight so I can comfortably wear it and my spex.

Light, takes little room in your pocket, stops the wind, breathes a fair bit.
Took it and a Power Stretch beanie to a few days' hike in November (0-10C, sometimes snowy, sometimes windy), ended up wearing this one all the time.

How can you go wrong with Mountain Hardwear and Windstopper? I use this on my winter runs, and it has performed admirably. No complaints here.