I recently started a very warm hat from Elizabeth Zimmermann's Knitting Around. You basically knit two hats that are attached to each other so you have a reversible double layer hat. I'm making this in the same yarn I used for my bias rib scarf, so I'll have a matching set.
The yarn is dramatically pooling and making spiral stripes on the first hat, I've decided I like it. I'll experiment with the other hat though, taking yarn from both ends of the ball to try and prevent pooling.