Little Knits has Opal sock yarn for 50-68% off. They still have a wide selection of colors left, including solids and 6-ply.
I bought two balls of the 6ply solids (mocha and chocolate) and two of the 4ply solids (cream and light blue). Because sometimes you don't want your socks to scream at your co-workers.
Just sayin' is all. =)
P.S. They also have many other yarns at reduced prices, if socks aren't your thing.