I almost always wear ultralight smartwool running socks:

http://www.rei.com/online/store/Prod...cat=REI_SEARCH

They are super thin and light. Never get too hot.

Those tetra socks are wild. They sponsor the FC 508, and a lot of ultra endurance folks love them. I don't know if I owuld like the material between the toes, though. Hmm.