My sister can't wear wool so I've had to search for alternatives to make her socks.
I'd have to say that the best non-wool yarn I've found so far is Greenwood Fiberworks cotton/lyrca sock yarn. It's thinner than Cascade Fixation.
For me it knits up at a gauge of 9spi. It's totally soft and lovely.
You can find it here:http://www.etsy.com/shop.php?user_id=19807
There are usually many more colorways (lots of pretty variegateds as well as solids) to choose from. I'd say if you don't see any that appeal to you now, check back in a couple of days.
I have some pictures of a sock I started using it and some more info about my impressions of the yarn on my blog.
Also, one of my favorite bloggers did a very very informative post about her experimentation with a huge variety of different non wool sock yarns. You can find her post about it here:http://www.grumperina.com/knitblog/nonwoolysockyarn.htm