Pasted from my recommended clothing page
Conventional wisdom for people wearing hiking boots is to wear a heavy wool sock, with an very light weight inner liner. Dr. Murray Hamlet of the Army Cold Weather Research Center confirmed this basic idea with an extensive testing of a variety of sock systems. A thin liner sock made of CoolMax, and a thicker sock made of wool&polypro (fuzzy side out) reduced the likelihood of blisters by a factor of three! This is because the slipping is happening between the socks rather than between the foot and the sock. TechSpun sells the socks designed by Hamlet. Besides TechSpun there are a variety of companies which sell high quality wool or wool blend backpacking socks including Bridgeport, Dahlgren, Darn Tough Socks, Fox River, SmartWool, Thorlo, Wigwam.
People carrying lighter loads and using trail runners or walking shoes often prefer socks that have less bulk. Since lighter shoes tend to flex with the foot, an inner / outer sock combination isn't as critical as when using boots. Some people find double socks are helpful with trail runners. Some low bulk ways to get double socks include using two pairs of nylon dress socks, or my favorite, WrightSocks double socks. WrightSocks can be found with both layers made from coolmax (best for hot weather), or coolmax with a wool outer (more durable and a bit warmer).
Single socks I would recommend for trail runners include Smartwool Adrenaline (wool), Inov-8 Mudsoc (coolmax), and the Bridgedale X-Hale Multisport Socks (multi-fabric blend).
People who have problems with blisters between their toes (like me) often use Injinji toe socks.
X-Socks look interesting but expensive, but I have no experience with them. See later sections on dealing with water and cold for additional options. Keep in mind that there needs to be enough room in your shoe for your feet and the socks you select. Socks which are too thick for your shoes will crowd your feet and result in blisters just as quickly as your feet sliding around. I would strongly recommend trying a variety of different socks and figure out what works best for you. Socks are cheap and will often mean the difference between a miserable blister plagued trip, and one with comfortable feet.