If it's truly cold, I wear DeFeet wool socks. Any other time, I wear whatever cheap 12 pack of black ankle socks are on sale at Target or the like. I used to spend stupid amounts of money on cycling specific socks, until I realized that any sock will work just fine. I'm not sure why people are so against cotton- I have never had any issues with wet feet from sweat, or stinky shoes- and I live in a warm climate, and I ride a lot.

As an added bonus, when I commute, I don't have to change socks when I get dressed.