Skora hoodie, thin wicking jersey material. Good enough for our weather down into the 30s. I hardly notice I'm wearing the hood under my helmet -- doesn't muffle sound or add thickness/bulk. I'd be wearing a long sleeve wicking baselayer anyway. The hoodie is convenient.

Into the 20s I have neoprene ear muffs, very thin, coiled spring, fits under the helmet or a pocket without bulk. Less than $10 from Walgreen's, which occasionally has surprisingly good winter apparel at very reasonable prices.

And a Nike Dri Fit thermal hoodie for casual rides. A bit thicker under the helmet, nothing a few strap adjustments can't handle.