Rowan Calmer, a 75% cotton, 25% microfiber yarn. The swatch pictured above is the color Coral, though it is more of an orange-red and was captured as more magenta-y than it is in person. The microfiber lightens the cotton considerably and gives stretch, making it pretty much the most comfortable yarn I've ever knit with. I haven't made socks with it yet, but I think it would make some wonderful arch-hugging and cushy socks. Normally this yarn knits at 5.25 sts per inch on size 8 US needles, the stretchiness means it can be knitted on bigger needles and come out with a tension similar to DK. The Rowan Calmer Collection book has a pattern for socks (called Mood, pictured below) knit on size 6 US needles, achieving 6 spi, or a 48-stitch sock. Each 50g ball is 160m/175yds (check out that ratio, I'm telling you this stuff is very light, not like pure cotton at all). Even though the ballband says to handwash, I've had success machine washing and drying an item I made with this yarn.
Jannette's Rare Yarns who is selling some of the older colorways at US$6.25-6.50 and shipping to the US (which is by air, i.e. fast) is $4 for the first ball, $1 for each additional. You might be interested to know there's a cute pattern for ballet slippers called Twinkletoes using Calmer, which is kind of pushing the definition of socks but hey they're footwear so I think it's relevant. :)