I wear smartwool socks year round - the lightweight or midweight in the summers. Not a big deal. I get the size small, but have size 6-6.5 feet that are on the wide side and they fit well.

+1 to not having your boots or shoes on too tight.

I wear my merrell chill slides a lot in the winter without socks. The fuzziness inside is much like uggs, but not full height.

+1 on keens like Becky said, although if you have thin feet you may not like them so much because they do have a wide toe box.